title
stringlengths 4
245
| abstract
stringlengths 26
3.26k
|
|---|---|
Local Lyman Break Galaxy Analogs: The Impact of Massive Star-forming
Clumps on the Interstellar Medium and the Global Structure of Young, Forming
Galaxies | We present HST UV/optical imaging, Spitzer mid-IR photometry, and optical
spectroscopy of a sample of 30 low-redshift (z=0.1-0.3) galaxies chosen from
SDSS/GALEX to be accurate local analogs of the high-z Lyman Break Galaxies. The
Lyman Break Analogs (LBAs) are similar in mass, metallicity, dust, SFR, size
and gas velocity dispersion, thus enabling a detailed investigation of
processes that are important at high-z. The optical emission line properties of
LBAs are also similar to those of LBGs, indicating comparable conditions in
their ISM. In the UV, LBAs are characterized by complexes of massive
star-forming "clumps", while in the optical they most often show evidence for
(post-)mergers/interactions. In 6 cases, we find an extremely massive (>10^9
Msun) compact (R~100 pc) dominant central object (DCO). The DCOs are
preferentially found in LBAs with the highest mid-IR luminosities and
correspondingly high SFRs (15-100 Msun/yr). We show that the massive SF clumps
(including the DCOs) have masses much larger than the nuclear super star
clusters seen in normal late type galaxies. However, the DCOs have masses,
sizes, and densities similar to the excess-light/central-cusps seen in typical
elliptical galaxies with masses similar to the LBA galaxies. We suggest that
the DCOs form in present-day examples of the dissipative mergers at high
redshift that are believed to have produced the central-cusps in local
ellipticals. More generally, the properties of the LBAs are consistent with the
idea that instabilities in a gas-rich disk lead to very massive star-forming
clumps that eventually coalesce to form a spheroid. We speculate that the DCOs
are too young at present to be growing a supermassive black hole because they
are still in a supernova-dominated outflow phase.
|
Spherical Collapse Models with Clustered Dark Energy | We investigate the clustering effect of dark energy (DE) in the formation of
galaxy clusters using the spherical collapse model. Assuming a fully clustered
DE component, the spherical overdense region is treated as an isolated system
which conserves the energy separately for both matter and DE inside the
spherical region. Then, by introducing a parameter $r$ to characterize the
degree of DE clustering, which is defined by the nonlinear density contrast
ratio of matter to DE at turnaround in the recollapsing process, i.e. $r\equiv
\nld_{\de,\ta}/\nld_{\m,\ta}$, we are able to uniquely determine the spherical
collapsing process and hence obtain the virialized overdensity $\Dvir$ through
a proper virialization scheme. Estimation of the virialized overdensities from
current observation on galaxy clusters suggests that $0.5 < r < 0.8$ at
$1\sigma$ level for the clustered DE with $w < -0.9$. Also, we compare our
method to the linear perturbation theory that deals with the growth of DE
perturbation at early times. While both results are consistent with each other,
our method is practically simple and it shows that the collapse process is
rather independent of initial DE perturbation and its evolution at early times.
|
Jets from MRC 0600-399 bent by magnetic fields in the cluster Abell 3376 | Galaxy clusters are known to harbour magnetic fields. The nature of the
intra-cluster magnetic fields remains an unresolved question. Intra-cluster
magnetic field can be observed at the density contact discontinuity formed by
cool and dense plasma running into hot ambient plasma, and the discontinuity
exists near the 2nd BCG MRC 0600-399 of a merging galaxy cluster Abell 3376
(z=0.0461, hereafter as A3376). Elongated X-ray image in the east-west
direction with a comet-like structure reaches a Mpc-scale (Fig1.(a)). Previous
radio observations detected the bent jets from MRC 0600-399, moving in same
direction as the sub-cluster's motion against ram pressure.Here we report a new
radio observation of a radio galaxy MRC 0600-399 which is 3.4 times and 11
times higher resolution and sensitivity than the previous results. Contrary to
typical jets, the MRC 0600-399 shows a 90deg bend at the contact discontinuity
and the collimated jets further extend over 100 kpc from the bend point.
Diffuse, elongated emission named "double-scythe" structures were detected for
the first time. The spectral index flattens downstream of the bend point,
indicating cosmic-ray re-acceleration. High-resolution numerical simulations
reveal that the ordered magnetic field along the discontinuity plays a
significant role in the change in the jet direction. The morphology of the
"double-scythe" bear remarkable similarities with the simulations, which
strengthens our understanding of the interaction between relativistic electrons
and intra-cluster magnetic field.
|
Axino Cold Dark Matter Revisited | Axino arises in supersymmetric versions of axion models and is a natural
candidate for cold or warm dark matter. Here we revisit axino dark matter
produced thermally and non-thermally in light of recent developments. First we
discuss the definition of axino relative to low energy axion one for several
KSVZ and DFSZ models of the axion. Then we review and refine the computation of
the dominant QCD production in order to avoid unphysical cross-sections and,
depending on the model, to include production via SU(2) and U(1) interactions
and Yukawa couplings.
|
Could the Magellanic Clouds be tidal dwarves expelled from a past-merger
event occurring in Andromeda? | The Magellanic Clouds are often considered as outliers in the satellite
system of the Milky Way because they are irregular and gas-rich galaxies. From
their large relative motion, they are likely from their first pass near the
Milky Way, possibly originating from another region of the Local Group or its
outskirts. M31 could have been in a merger stage in its past and we investigate
whether or not the Large Magellanic Cloud could have been a tidal dwarf
expelled during this event. Such an hypothesis is tested in the frame of
present-day measurements and uncertainties of the relative motions of LMC and
M31. Our method is to trace back the LMC trajectory using several thousands of
different configurations that sample the corresponding parameter space. We find
several configurations that let LMC at 50 kpc from M31, 4.3 to 8 Gyrs ago,
depending on the adopted shape of the Milky Way halo. For all configurations,
the LMC velocity at such a location is invariably slightly larger than the
escape velocity at such a radius. The preferred solutions correspond to a
spherical to prolate Milky Way halo, predicting a transversal motion of M31 of
less than 107 km/s and down to values that are close to zero. We conclude that
from present-day measurements, Magellanic Clouds could well be tidal dwarves
expelled from a former merger events occurring in M31.
|
Update constraints on neutrino mass and mass hierarchy in light of dark
energy models | Combining cosmic microwave background (CMB) data from Planck satellite data,
Baryon Acoustic Oscillations (BAO) measurements and Type Ia supernovae (SNe Ia)
data, we obtain the bounds on total neutrino masses $M_\nu$ with the
approximation of degenerate neutrino masses and for three dark energy models:
the cosmological constant ($\Lambda$CDM) model, a phenomenological emergent
dark energy (PEDE) model and a model-independent quintessential
parameterization (HBK). The bounds on the sum of neutrino masses $M_\nu$ depend
on the dark energy (DE) models. In the HBK model, we confirm the conclusion
from some previous work that the quintessence prior of dark energy tends to
tighten the cosmological constraint on $M_\nu$. On the other hand, the PEDE
model leads to larger $M_\nu$ and a nonzero lower bound. Besides, we also
explore the correlation between three different neutrino hierarchies and dark
energy models.
|
The three-year shear catalog of the Subaru Hyper Suprime-Cam SSP Survey | We present the galaxy shear catalog that will be used for the three-year
cosmological weak gravitational lensing analyses using data from the Wide layer
of the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) Survey. The
galaxy shapes are measured from the $i$-band imaging data acquired from 2014 to
2019 and calibrated with image simulations that resemble the observing
conditions of the survey based on training galaxy images from the Hubble Space
Telescope in the COSMOS region. The catalog covers an area of 433.48 deg$^2$ of
the northern sky, split into six fields. The mean $i$-band seeing is 0.59
arcsec. With conservative galaxy selection criteria (e.g., $i$-band magnitude
brighter than 24.5), the observed raw galaxy number density is 22.9
arcmin$^{-2}$, and the effective galaxy number density is 19.9 arcmin$^{-2}$.
The calibration removes the galaxy property-dependent shear estimation bias to
a level: $|\delta m|<9\times 10^{-3}$. The bias residual $\delta m$ shows no
dependence on redshift in the range $0<z\leq 3$. We define the requirements for
cosmological weak lensing science for this shear catalog, and quantify
potential systematics in the catalog using a series of internal null tests for
systematics related to point-spread function modelling and shear estimation. A
variety of the null tests are statistically consistent with zero or within
requirements, but (i) there is evidence for PSF model shape residual
correlations; and (ii) star-galaxy shape correlations reveal additive
systematics. Both effects become significant on $>1$ degree scales and will
require mitigation during the inference of cosmological parameters using cosmic
shear measurements.
|
Galilean Genesis: an alternative to inflation | We propose a novel cosmological scenario, in which standard inflation is
replaced by an expanding phase with a drastic violation of the Null Energy
Condition (NEC): \dot H >> H^2. The model is based on the recently introduced
Galileon theories, that allow NEC violating solutions without instabilities.
The unperturbed solution describes a Universe that is asymptotically Minkowski
in the past, expands with increasing energy density until it exits the regime
of validity of the effective field theory and reheats. This solution is a
dynamical attractor and the Universe is driven to it, even if it is initially
contracting. The study of perturbations of the Galileon field reveals some
subtleties, related to the gross violation of the NEC and it shows that
adiabatic perturbations are cosmologically irrelevant. The model, however,
suggests a new way to produce a scale invariant spectrum of isocurvature
perturbations, which can later be converted to adiabatic: the Galileon is
forced by symmetry to couple to the other fields as a dilaton; the effective
metric it yields on the NEC violating solution is that of de Sitter space, so
that all light scalars will automatically acquire a nearly scale-invariant
spectrum of perturbations.
|
The Second Survey of the Molecular Clouds in the Large Magellanic Cloud
by NANTEN. II. Star Formation | We studied star formation activities in the molecular clouds in the Large
Magellanic Cloud. We have utilized the second catalog of 272 molecular clouds
obtained by NANTEN to compare the cloud distribution with signatures of massive
star formation including stellar clusters, and optical and radio HII regions.
We find that the molecular clouds are classified into three types according to
the activities of massive star formation; Type I shows no signature of massive
star formation, Type II is associated with relatively small HII region(s) and
Type III with both HII region(s) and young stellar cluster(s). The radio
continuum sources were used to confirm that Type I GMCs do not host optically
hidden HII regions. These signatures of massive star formation show a good
spatial correlation with the molecular clouds in a sense they are located
within ~100 pc of the molecular clouds. Among possible ideas to explain the GMC
Types, we favor that the Types indicate an evolutionary sequence; i.e., the
youngest phase is Type I, followed by Type II and the last phase is Type III,
where the most active star formation takes place leading to cloud dispersal.
The number of the three types of GMCs should be proportional to the time scale
of each evolutionary stage if a steady state of massive star and cluster
formation is a good approximation. By adopting the time scale of the youngest
stellar clusters, 10 Myrs, we roughly estimate the timescales of Types I, II
and III to be 6 Myrs, 13 Myrs and 7 Myrs, respectively, corresponding to a
lifetime of 20-30 Myrs for the GMCs with a mass above the completeness limit, 5
x 10^4 Msun.
|
Non-linear Evolution of Matter Power Spectrum in Modified Theory of
Gravity | We present a formalism to calculate the non-linear matter power spectrum in
modified gravity models that explain the late-time acceleration of the Universe
without dark energy. Any successful modified gravity models should contain a
mechanism to recover General Relativity (GR) on small scales in order to avoid
the stringent constrains on deviations from GR at solar system scales. Based on
our formalism, the quasi non-linear power spectrum in the
Dvali-Gabadadze-Porratti (DGP) braneworld models and $f(R)$ gravity models are
derived by taking into account the mechanism to recover GR properly. We also
extrapolate our predictions to fully non-linear scales using the Parametrized
Post Friedmann (PPF) framework. In $f(R)$ gravity models, the predicted
non-linear power spectrum is shown to reproduce N-body results. We find that
the mechanism to recover GR suppresses the difference between the modified
gravity models and dark energy models with the same expansion history, but the
difference remains large at weakly non-linear regime in these models. Our
formalism is applicable to a wide variety of modified gravity models and it is
ready to use once consistent models for modified gravity are developed.
|
The AGN Black Hole Mass Database | The AGN Black Hole Mass Database is a compilation of all published
spectroscopic reverberation-mapping studies of active galaxies. We have created
a public web interface, where users may get the most up-to-date black hole
masses from reverberation mapping for any particular active galactic nucleus
(AGN), as well as obtain the individual measurements upon which the masses are
based and the appropriate references. While the database currently focuses on
the measurements necessary for black hole mass determinations, we also plan to
expand it in the future to include additional useful information, such as
host-galaxy characteristics. New reverberation mapping results will also be
incorporated into the database as they are published in peer-refereed journals.
|
153 MHz GMRT follow-up of steep-spectrum diffuse emission in galaxy
clusters | In this paper we present new high sensitivity 153 MHz Giant Meterwave Radio
Telescope follow-up observations of the diffuse steep spectrum cluster radio
sources in the galaxy clusters Abell 521, Abell 697, Abell 1682. Abell 521
hosts a relic, and together with Abell 697 it also hosts a giant very steep
spectrum radio halo. Abell 1682 is a more complex system with candidate steep
spectrum diffuse emission. We imaged the diffuse radio emission in these
clusters at 153 MHz, and provided flux density measurements of all the sources
at this frequency. Our new flux density measurements, coupled with the existing
data at higher frequencies, allow us to study the total spectrum of the halos
and relic over at least one order of magnitude in frequency. Our images confirm
the presence of a very steep "diffuse component" in Abell 1682. We found that
the spectrum of the relic in Abell 521 can be fitted by a single power-law with
$\alpha=1.45\pm0.02$ from 153 MHz to 5 GHz. Moreover, we confirm that the halos
in Abell 521 and Abell 697 have a very steep spectrum, with $\alpha=1.8-1.9$
and $\alpha=1.52\pm0.05$ respectively. Even with the inclusion of the 153 MHz
flux density information it is impossible to discriminate between power-law and
curved spectra, as derived from homogeneous turbulent re-acceleration. The
latter are favored on the basis of simple energetic arguments, and we expect
that LOFAR will finally unveil the shape of the spectra of radio halos below
100 MHz, thus providing clues on their origin.
|
A Case for Radio Galaxies as the Sources of IceCube's Astrophysical
Neutrino Flux | We present an argument that radio galaxies (active galaxies with mis-aligned
jets) are likely to be the primary sources of the high-energy astrophysical
neutrinos observed by IceCube. In particular, if the gamma-ray emission
observed from radio galaxies is generated through the interactions of
cosmic-ray protons with gas, these interactions can also produce a population
of neutrinos with a flux and spectral shape similar to that measured by
IceCube. We present a simple physical model in which high-energy cosmic rays
are confined within the volumes of radio galaxies, where they interact with gas
to generate the observed diffuse fluxes of neutrinos and gamma rays. In
addition to simultaneously accounting for the observations of Fermi and
IceCube, radio galaxies in this model also represent an attractive class of
sources for the highest energy cosmic rays.
|
The MWA GLEAM 4-Jy (G4Jy) Sample | Powerful radio-galaxies feature heavily in our understanding of galaxy
evolution. However, when it comes to studying their properties as a function of
redshift and/or environment, the most-detailed studies tend to be limited by
small-number statistics. During Focus Meeting 3, on "Radio Galaxies: Resolving
the AGN phenomenon", SVW presented a new sample of nearly 2,000 of the
brightest radio-sources in the southern sky (Dec. $<$ 30 deg). These were
observed at low radio-frequencies as part of the GaLactic and Extragalactic
All-sky MWA (GLEAM) Survey, which is a continuum survey conducted using the
Murchison Widefield Array (MWA). This instrument is the precursor telescope for
the low-frequency component of the Square Kilometre Array, and allows us to
select radio galaxies in an orientation-independent way (i.e. minimising the
bias caused by Doppler boosting, inherent in high-frequency surveys). Being
brighter than 4 Jy at 151 MHz, we refer to these objects as the GLEAM 4-Jy
(G4Jy) Sample. The G4Jy catalogue is close to being finalised, with SVW
describing how multi-wavelength data have been used to determine the morphology
of the radio emission, and identify the host galaxy. In addition, the MWA's
excellent spectral-coverage and sensitivity to extended/diffuse emission were
highlighted. Both of these aspects are important for understanding the physical
mechanisms that take place within active galaxies, and how they interact with
their environment.
|
Dark matter repulsion could thwart direct detection | We consider a feeble repulsive interaction between ordinary matter and dark
matter, with a range similar to or larger than the size of the Earth. Dark
matter can thus be repelled from the Earth, leading to null results in direct
detection experiments, regardless of the strength of the short-distance
interactions of dark matter with atoms. Generically, such a repulsive force
would not allow trapping of dark matter inside astronomical bodies. In this
scenario, accelerator-based experiments may furnish the only robust signals of
asymmetric dark matter models, which typically lack indirect signals from
self-annihilation. Some of the variants of our hypothesis are also briefly
discussed.
|
Star formation and molecular hydrogen in dwarf galaxies: a
non-equilibrium view | We study the connection of star formation to atomic (HI) and molecular
hydrogen (H$_2$) in isolated, low metallicity dwarf galaxies with
high-resolution ($m_{\rm gas}$ = 4 M$_\odot$, $N_{\rm ngb}$ = 100) SPH
simulations. The model includes self-gravity, non-equilibrium cooling,
shielding from an interstellar radiation field, the chemistry of H$_2$
formation, H$_2$-independent star formation, supernova feedback and metal
enrichment. We find that the H$_2$ mass fraction is sensitive to the adopted
dust-to-gas ratio and the strength of the interstellar radiation field, while
the star formation rate is not. Star formation is regulated by stellar
feedback, keeping the gas out of thermal equilibrium for densities $n <$ 1
cm$^{-3}$. Because of the long chemical timescales, the H$_2$ mass remains out
of chemical equilibrium throughout the simulation. Star formation is
well-correlated with cold ( T $\leqslant$ 100 K ) gas, but this dense and cold
gas - the reservoir for star formation - is dominated by HI, not H$_2$. In
addition, a significant fraction of H$_2$ resides in a diffuse, warm phase,
which is not star-forming. The ISM is dominated by warm gas (100 K $<$ T
$\leqslant 3\times 10^4$ K) both in mass and in volume. The scale height of the
gaseous disc increases with radius while the cold gas is always confined to a
thin layer in the mid-plane. The cold gas fraction is regulated by feedback at
small radii and by the assumed radiation field at large radii. The decreasing
cold gas fractions result in a rapid increase in depletion time (up to 100
Gyrs) for total gas surface densities $\Sigma_{\rm HI+H_2} \lesssim$ 10
M$_\odot$pc$^{-2}$, in agreement with observations of dwarf galaxies in the
Kennicutt-Schmidt plane.
|
Proper Motions of H-alpha filaments in the Supernova Remnant RCW 86 | We present a proper motion study of the eastern shock-region of the supernova
remnant RCW 86 (MSH 14-63, G315.4-2.3), based on optical observations carried
out with VLT/FORS2 in 2007 and 2010. For both the northeastern and southeastern
regions, we measure an average proper motion of H-alpha filaments of 0.10 +/-
0.02 arcsec/yr, corresponding to 1200 +/- 200 km/s at 2.5kpc. There is
substantial variation in the derived proper motions, indicating shock
velocities ranging from just below 700 km/s to above 2200 km/s.
The optical proper motion is lower than the previously measured X-ray proper
motion of northeastern region. The new measurements are consistent with the
previously measured proton temperature of 2.3 +/- 0.3 keV, assuming no
cosmic-ray acceleration. However, within the uncertainties, moderately
efficient (< 27 per cent) shock acceleration is still possible. The combination
of optical proper motion and proton temperature rule out the possibility that
RCW 86 has a distance less than 1.5kpc.
The similarity of the proper motions in the northeast and southeast is
peculiar, given the different densities and X-ray emission properties of the
regions. The northeastern region has lower densities and the X-ray emission is
synchrotron dominated, suggesting that the shock velocities should be higher
than in the southeastern, thermal X-ray dominated, region. A possible solution
is that the H-alpha emitting filaments are biased toward denser regions, with
lower shock velocities. Alternatively, in the northeast the shock velocity may
have decreased rapidly during the past 200yr, and the X-ray synchrotron
emission is an afterglow from a period when the shock velocity was higher.
|
A Method for Measuring (Slopes of) the Mass Profiles of Dwarf Spheroidal
Galaxies | We introduce a method for measuring the slopes of mass profiles within dwarf
spheroidal (dSph) galaxies directly from stellar spectroscopic data and without
adopting a dark matter halo model. Our method combines two recent results: 1)
spherically symmetric, equilibrium Jeans models imply that the product of
halflight radius and (squared) stellar velocity dispersion provides an estimate
of the mass enclosed within the halflight radius of a dSph stellar component,
and 2) some dSphs have chemo-dynamically distinct stellar
\textit{sub}components that independently trace the same gravitational
potential. We devise a statistical method that uses measurements of stellar
positions, velocities and spectral indices to distinguish two dSph stellar
subcomponents and to estimate their individual halflight radii and velocity
dispersions. For a dSph with two detected stellar subcomponents, we obtain
estimates of masses enclosed at two discrete points in the same mass profile,
immediately defining a slope. Applied to published spectroscopic data, our
method distinguishes stellar subcomponents in the Fornax and Sculptor dSphs,
for which we measure slopes $\Gamma\equiv \Delta \log M / \Delta \log
r=2.61_{-0.37}^{+0.43}$ and $\Gamma=2.95_{-0.39}^{+0.51}$, respectively. These
values are consistent with 'cores' of constant density within the central
few-hundred parsecs of each galaxy and rule out `cuspy' Navarro-Frenk-White
(NFW) profiles ($d\log M/d\log r \leq 2$ at all radii) with significance $\ga
96%$ and $\ga 99%$, respectively. Tests with synthetic data indicate that our
method tends systematically to overestimate the mass of the inner stellar
subcomponent to a greater degree than that of the outer stellar subcomponent,
and therefore to underestimate the slope $\Gamma$ (implying that the stated NFW
exclusion levels are conservative).
|
XMM-Newton Survey of Local O VII Absorption Lines in the Spectra of
Active Galactic Nuclei | Highly ionized, z=0 metal absorption lines detected in the X-ray spectra of
background active galactic nuclei (AGNs) provide an effective method to probe
the hot ($T\sim10^6$ K) gas and its metal content in and around the Milky Way.
We present an all-sky survey of the $K_{\alpha}$ transition of the local O VII
absorption lines obtained by Voigt-profile fitting archival XMM-Newton
observations. A total of 43 AGNs were selected, among which 12 are BL Lac-type
AGNs, and the rest are Seyfert 1 galaxies. At above the $3\sigma$ level the
local O VII absorption lines were detected in 21 AGNs, among which 7 were newly
discovered in this work. The sky covering fraction, defined as the ratio
between the number of detections and the sample size, increases from at about
40% for all targets to 100% for the brightest targets, suggesting a uniform
distribution of the O VII absorbers. We correlate the line equivalent width
with the Galactic coordinates and do not find any strong correlations between
these quantities. Some AGNs have warm absorbers that may complicate the
analysis of the local X-ray absorber since the recession velocity can be
compensated by the outflow velocity, especially for the nearby targets. We
discuss the potential impact of the warm absorbers on our analysis. A
comprehensive theoretical modelling of the X-ray absorbers will be presented in
a later paper.
|
Imaging an Event Horizon: Mitigation of Scattering Toward Sagittarius A* | The image of the emission surrounding the black hole in the center of the
Milky Way is predicted to exhibit the imprint of general relativistic (GR)
effects, including the existence of a shadow feature and a photon ring of
diameter ~50 microarcseconds. Structure on these scales can be resolved by
millimeter-wavelength very long baseline interferometry (VLBI). However,
strong-field GR features of interest will be blurred at lambda >= 1.3 mm due to
scattering by interstellar electrons. The scattering properties are well
understood over most of the relevant range of baseline lengths, suggesting that
the scattering may be (mostly) invertible. We simulate observations of a model
image of Sgr A* and demonstrate that the effects of scattering can indeed be
mitigated by correcting the visibilities before reconstructing the image. This
technique is also applicable to Sgr A* at longer wavelengths.
|
Are all perturbations created equal? An analysis of the WMAP 5- and
7-year data without inflationary prejudice | We submit recent claims of a semi-significant detection of primordial tensor
perturbations in the WMAP data to a closer scrutiny. Our conclusion is in brief
that no such mode is present at a detectable level once the analysis is done
more carefully. These claims have their root in a brief debate in the late
1990s about the standard calculation of the scalar and tensor spectra in
standard inflationary theory, where Grishchuk and collaborators claimed that
their amplitudes should be roughly equal. We give a brief summary of the debate
and our own reasons for why the standard calculation is correct.
|
The Mass Function of Unprocessed Dark Matter Halos and Merger Tree
Branching Rates | A common approach in semi-analytic modeling of galaxy formation is to
construct Monte Carlo realizations of merger histories of dark matter halos
whose masses are sampled from a halo mass function. Both the mass function
itself, and the merger rates used to construct merging histories are calibrated
to N-body simulations. Typically, "backsplash" halos (those which were once
subhalos within a larger halo, but which have since moved outside of the halo)
are counted in both the halo mass function, and in the merger rates (or,
equivalently, progenitor mass functions). This leads to a double-counting of
mass in Monte Carlo merger histories which will bias results relative to N-body
results. We measure halo mass functions and merger rates with this
double-counting removed in a large, cosmological N-body simulation with
cosmological parameters consistent with current constraints. Furthermore, we
account for the inherently noisy nature of N-body halo mass estimates when
fitting functions to N-body data, and show that ignoring these errors leads to
a significant systematic bias given the precision statistics available from
state-of-the-art N-body cosmological simulations.
|
Anthropic Argument for Three Generations | The standard model of particle physics contains N_gen=3 generations of quarks
and leptons, i.e., two sets of three particles in each sector, with the two
sets differing by 1 unit of charge in each. All 12 "predicted" particles are
now experimentally accounted for, and there are strong (though not air-tight)
arguments that there are no more than three generations. The question is: why
exactly N_gen=3? I argue that three generations is a natural prediction of the
multiverse theory, provided one adds the additional, quite reasonable
assumption that N_gen in a randomly realized universe is a steeply falling
function of number. In this case N_gen > 2 to permit CP violation (and so
baryogenesis and thus physicists) and N_gen < 4 to avoid highly improbable
outcomes. I thereby make a testable anthropic-principle prediction: that when a
theory of randomly realized N_gen is developed, the probability will turn out
to be steeply falling in N_gen.
|
Reionization on Large Scales III: Predictions for Low-ell Cosmic
Microwave Background Polarization and High-ell Kinetic Sunyaev-Zel'dovich
Observables | We present new predictions for temperature (on small angular scales) and
polarization (on large angular scales) CMB anisotropies induced during the
epoch of reionization (EoR). Using a novel method calibrated from
Radiation-Hydrodynamic simulations we model the EoR in large volumes (L >~ 2
Gpc/h) in the context of galactic reionization. We find that the EoR
contribution to the kinetic Sunyaev- Zel'dovich power spectrum (patchy kSZ)
ranges between ~0.6 - 2.8 muK^2 at ell = 3000, for the parameter space we
explored. These patchy kSZ power spectra are calculated from large 15 Deg x 15
Deg maps that are found to be necessary. Decreasing the size of these maps
biases the overall patchy kSZ power to higher values. We find that the
amplitude of the patchy kSZ power spectrum at ell = 3000 follows simple
scalings of D_ell=3000^kSZ propto <z> and D_ell=3000^kSZ propto Delz^0.47 for
the mean redshift (<z>) of reionization and the duration (dz). Using the
constraints on <z> from WMAP 7-year results and the lower limit on dz from
EDGES we find a lower limit of ~ 0.4 muK^2 on the kSZ at ell = 3000. Planck
will constrain the mean redshift and the Thomson optical depth from the low-ell
polarization power spectrum. Future measurements of the high-ell CMB power
spectrum from the South Pole Telescope (SPT) and the Atacama Cosmology
Telescope (ACT) should detect the patchy kSZ signal if the cross correlation
between the cosmic infrared background and the thermal Sunyaev Zel'dovich
effect is constrained. We show that the combination of temperature and
polarization measurements constrains both <z> and dz. The patchy kSZ maps,
power spectra templates and the polarization power spectra will be publicly
available.
|
The SDSS-DR12 large-scale cross-correlation of Damped Lyman Alpha
Systems with the Lyman Alpha Forest | We present a measurement of the DLA mean bias from the cross-correlation of
DLA and the Ly$\alpha$ forest, updating earlier results of Font-Ribera et al.
2012 with the final BOSS Data Release and an improved method to address
continuum fitting corrections. Our cross-correlation is well fitted by linear
theory with the standard $\Lambda CDM$ model, with a DLA bias of $b_{\rm DLA} =
1.99\pm 0.11$; a more conservative analysis, which removes DLA in the Ly$\beta$
forest and uses only the cross-correlation at $r> 10{\rm h^{-1}\,Mpc}$, yields
$b_{\rm DLA} = 2.00\pm 0.19$. This assumes the cosmological model from
\cite{Planck2015} and the Ly$\alpha$ forest bias factors of Bautista et al.
2017, and includes only statistical errors obtained from bootstrap analysis.
The main systematic errors arise from possible impurities and selection effects
in the DLA catalogue, and from uncertainties in the determination of the
Ly$\alpha$ forest bias factors and a correction for effects of high column
density absorbers. We find no dependence of the DLA bias on column density or
redshift. The measured bias value corresponds to a host halo mass $\sim
4\cdot10^{11} {\rm M_{\odot}}$ if all DLA were hosted in halos of a similar
mass. In a realistic model where host halos over a broad mass range have a DLA
cross section $\Sigma(M_h) \propto M_h^{\alpha}$ down to $M_h > M_{\rm min}
=10^{8.5} {\rm M_{\odot}}$, we find that $\alpha > 1$ is required to have
$b_{\rm DLA}> 1.7$, implying a steeper relation or higher value of $M_{\rm
min}$ than is generally predicted in numerical simulations of galaxy formation.
|
The Inner Halo of M87: A First Direct View of the Red-Giant Population | An unusually deep (V,I) imaging dataset for the Virgo supergiant M87 with the
Hubble Space Telescope ACS successfully resolves its brightest red-giant stars,
reaching M_I(lim) = -2.5. After assessing the photometric completeness and
biasses, we use this material to estimate the metallicity distribution for the
inner halo of M87, finding that the distribution is very broad and likely to
peak near [m/H] ~ -0.4 and perhaps higher. The shape of the MDF strongly
resembles that of the inner halo for the nearby giant E galaxy NGC 5128. As a
byproduct of our study, we also obtain a preliminary measurement of the
distance to M87 with the TRGB (red-giant branch tip) method; the result is
(m-M)_0 = 31.12 +- 0.14 (d = 16.7 +- 0.9 Mpc). Averaging this result with three
other recent techniques give a weighted mean d(M87) = (16.4 +- 0.5) Mpc.
|
Combining FOF and halo-based algorithms for the identification of galaxy
groups | Galaxy groups provide the means for a great diversity of studies that
contribute to a better understanding of the structure of the universe on a
large scale and allow the properties of galaxies to be linked to those of the
host halos. However, the identification of galaxy systems is a challenging task
and therefore it is necessary to improve the techniques involved as much as
possible. In view of the large present and forthcoming galaxy catalogues, we
propose, implement, and evaluate an algorithm that combines the two most
popular techniques used to identify galaxy systems. The algorithm can be easily
applied to any spectroscopic galaxy catalogue, but here we demonstrate its use
on the Sloan Digital Sky Survey. Assuming that a galaxy group is a
gravitationally bounded system that has at least one bright galaxy, we begin by
identifying groups with a Friends-Of-Friends algorithm adapted to fit this
definition. In view of the shortcomings of this method, particularly the lack
of ability to identify low-number groups, and consequently the inability to
study the occupation of halos throughout the mass range, we improve it by
adding a halo-based procedure. To assess the performance, we construct a mock
catalogue from a semi-analytical model to compare the groups identified using
our method with those obtained from the simulation. The comparison of groups
extracted using our method with those of a mock catalogue shows that the
proposed algorithm provides excellent results. The modifications introduced to
the Friends-Of-Friends algorithm in the first part of the procedure to fit the
adopted group definition gave reliable groups. Furthermore, the incorporation
of the halo-based method reduces the interlopers while more accurately
reproducing the number of galaxies per group. As a specific application, we use
the algorithm to extract groups from the Sloan Digital Sky Survey.
|
Revealing the Ionization Properties of the Magellanic Stream using
Optical Emission | The Magellanic Stream, a gaseous tail that trails behind the Magellanic
Clouds, could replenish the Milky Way with a tremendous amount of gas if it
reaches the Galactic disk before it evaporates into the halo. To determine how
the Magellanic Stream's properties change along its length, we have conducted
an observational study of the H-alpha emission, along with other optical warm
ionized gas tracers, toward 39 sight lines. Using the Wisconsin H-alpha Mapper
telescope, we detect H-alpha emission brighter than 30 - 50 mR in 26 of our 39
sight lines. This H-alpha emission extends more than 2-degree away from the HI
emission. By comparing H-alpha and [OI] intensities, we find that regions with
log NHI = 19.5 - 20.0 are 16 - 67% ionized. Most of the H-alpha intensities
along the Magellanic Stream are much higher than expected if the primary
ionization source is photoionization from Magellanic Clouds, the Milky Way, and
the extragalactic background. We find that the additional contribution from
self ionization through a "shock cascade" that results as the Stream plows
through the halo might be sufficient to reproduce the underlying level of
H-alpha emission along the Stream. In the sparsely sampled region below the
South Galactic Pole, there exists a subset of sight lines with
uncharacteristically bright emission, which suggest that gas is being ionized
further by an additional source that could be a linked to energetic processes
associated with the Galactic center.
|
Terahertz Water Masers: II. Further SOFIA/GREAT Detections toward
Circumstellar Outflows, and a Multitransition Analysis | Following up on our discovery of terahertz water masers, reported in 2017, we
report two further detections of water maser emission at frequencies above 1
THz. Using the GREAT instrument on SOFIA, we have detected emission in the
1.296411 THz $8_{27}-7_{34}$ transition of water toward two additional
oxygen-rich evolved stars, omicron Ceti (Mira) and R Crateris, and obtained an
upper limit on the 1.296 THz line emission from U Orionis. Toward these three
sources, and toward the red supergiant star VY Canis Majorae from which 1.296
THz line emission was reported previously, we have also observed several
lower-frequency (sub)millimeter water maser transitions using the APEX 12-m
telescope along with the 22 GHz transition using the Effelsberg 100-m
telescope. We have used a simple model to analyse the multi-transition data
thereby obtained. Adopting, as a prior, independent literature estimates of the
mass-loss-rates in these four sources and in W Hydrae, we infer water
abundances in a remarkably narrow range: $n({\rm H_2O})/n({\rm H_2}) = 1.4 -
2.5 \times 10^{-4}$. For o Cet, VY CMa, and W Hya, the model is successful in
predicting the maser line fluxes to within a typical factor $\sim 1.6 - 3$. For
R Crt and U Ori, the model is less successful, with typical line flux
predictions lying an order of magnitude above or below the observations; such
discrepancies are perhaps unsurprising given the exponential nature of maser
amplification.
|
Cosmological constraints on ultra-light axion fields | Ultra-light axions (ULAs) with mass less than 10^-20 eV have interesting
behaviors that may contribute to either dark energy or dark matter at different
epochs of the Universe. Its properties can be explored by cosmological
observations, such as expansion history of the Universe, cosmic large-scale
structure, cosmic microwave background, etc. In this work, we study the ULAs
with a mass around 10^-33 eV, which means the ULA field still rolls slowly at
present with the equation of state w=-1 as dark energy. In order to investigate
the mass and other properties of this kind of ULA field, we adopt the
measurements of Type Ia supernova (SN Ia), baryon acoustic oscillation (BAO),
and Hubble parameter H(z). The Markov Chain Monte Carlo (MCMC) technique is
employed to perform the constraints on the parameters. Finally, by exploring
four cases of the model, we find that the mass of this ULA field is about
3x10^-33 eV if assuming the initial axion field phi_i=M_pl. We also investigate
a general case by assuming phi_i< M_pl and find that the fitting results of
phi_i/M_pl are consistent with or close to 1 for the datasets we use.
|
Deriving Stellar Effective Temperatures of Metal-Poor Stars with the
Excitation Potential Method | It is well established that stellar effective temperatures determined from
photometry and spectroscopy yield systematically different results. We describe
a new, simple method to correct spectroscopically derived temperatures
("excitation temperatures") of metal-poor stars based on a literature sample
with -3.3<[Fe/H]<-2.5. Excitation temperatures were determined from FeI line
abundances in high-resolution optical spectra in the wavelength range of ~3700
to ~7000A, although shorter wavelength ranges, up to 4750 to 6800A, can also be
employed, and compared with photometric literature temperatures. Our adjustment
scheme increases the temperatures up to several hundred degrees for cool red
giants, while leaving the near-main-sequence stars mostly unchanged. Hence, it
brings the excitation temperatures in good agreement with photometrically
derived values. The modified temperature also influences other stellar
parameters, as the FeI-FeII ionization balance is simultaneously used to
determine the surface gravity, while also forcing no abundance trend on the
absorption line strengths to obtain the microturbulent velocity. As a result of
increasing the temperature, the often too low gravities and too high
microturbulent velocities in red giants become higher and lower, respectively.
Our adjustment scheme thus continues to build on the advantage of deriving
temperatures from spectroscopy alone, independent of reddening, while at the
same time producing stellar chemical abundances that are more straightforwardly
comparable to studies based on photometrically derived temperatures. Hence, our
method may prove beneficial for comparing different studies in the literature
as well as the many high-resolution stellar spectroscopic surveys that are or
will be carried out in the next few years.
|
Rotation Curve of the Milky Way out to $\sim$ 200 kpc | The rotation curve (RC) of our Galaxy, the Milky Way, is constructed starting
from its very inner regions (few hundred pc) out to a large Galactocentric
distance of $\sim 200$ kpc using kinematical data on a variety of tracer
objects moving in the gravitational potential of the Galaxy, without assuming
any theoretical models of the visible and dark matter components of the Galaxy.
We study the effect on the RC due to the uncertainties in the values of the
Galactic Constants (GCs) $R_\odot$ and $V_\odot$ (these being the sun's
distance from and circular rotation speed around the Galactic center,
respectively) and the velocity anisotropy parameter $\beta$ of the halo tracer
objects used for deriving the RC at large Galactocentric distances. The
resulting RC in the disk region is found to depend significantly on the choice
of the GCs, while the dominant uncertainty in the RC at large distances beyond
the stellar disk comes from the uncertainty in the value of $\beta$. In general
we find that the mean RC steadily declines at distances beyond $\sim 60$ kpc,
independently of the value of $\beta$. Also, at a given radius, the circular
speed is lower for larger values of $\beta$ (i.e., for more radially biased
velocity anisotropy). Considering that the largest possible value of $\beta$ is
unity, which corresponds to stellar orbits being purely radial, our results for
the case of $\beta=1$ give a lower limit to the total mass of the Galaxy within
$\sim 200$ kpc, $M(200 {\rm kpc}) \gsim (6.8\pm4.1) \times 10^{11} M_\odot$,
independently of any model of the dark matter halo of the Galaxy.
|
The redshift evolution of the baryonic Tully-Fisher relation in Simba | The baryonic Tully-Fisher relation (BTFR) is an important tool for
constraining galaxy evolution models. As 21-cm HI emission studies have been
largely restricted to low redshifts, the redshift evolution of the BTFR is less
studied. The upcoming LADUMA survey (Looking At the Distant Universe with the
MeerKAT Array) will address this. As preparation for LADUMA, we use the Simba
hydrodynamical galaxy formation simulation from the Simba-hires (25 h$^{-1}$
Mpc)$^{3}$ run to generate rotational velocity measures from galaxy rotation
curves ($V_{\rm flat}$) and HI spectral line profile widths ($W_{\rm 50}$ and
$W_{\rm 20}$) at three different redshifts ($z$ = 0, 0.5, and 1). Using these
measures, together with the dark matter velocity dispersion and halo mass, we
consider the redshift evolution of the BTFR of Simba galaxies. We find that
LADUMA will be successful in detecting weak redshift evolution of the BTFR,
provided that auxiliary data is used to distinguish galaxies with disky
morphologies. $W_{\rm 20}$ spectral line widths give lower scatter and more
pronounced redshift evolution compared to $W_{\rm 50}$. We also compare these
rotational velocity measures to the dark matter velocity dispersion across
redshift and galaxy morphology. We find weak redshift evolution between
rotational velocity and the dark matter halo mass, and provide fits for
estimating a galaxy's dark matter halo mass from HI spectral line widths. This
study with Simba showcases the importance of upcoming, deep SKA pathfinder
surveys such as LADUMA, and provides predictions to compare with redshift
evolution of the BTFR and galaxy dark matter content from HI rotational
velocity measures.
|
Identifying and Repairing Catastrophic Errors in Galaxy Properties Using
Dimensionality Reduction | Our understanding of galaxy evolution is derived from large surveys designed
to maximize efficiency by only observing the minimum amount needed to infer
properties for a typical galaxy. However, for a few percent of galaxies in
every survey, these observations are insufficient and derived properties can be
catastrophically wrong. Further, it is currently difficult or impossible to
determine which objects have failed, so that these contaminate every study of
galaxy properties. We develop a novel method to identify these objects by
combining the astronomical codes which infer galaxy properties with the
dimensionality reduction algorithm t-SNE, which groups similar objects to
determine which inferred properties are out of place. This method provides an
improvement for the COSMOS catalog, which already uses existing techniques for
catastrophic error removal, and therefore should improve the quality of large
catalogs and any studies which are sensitive to large redshift errors.
|
Wind nebulae and supernova remnants of very massive stars | A very small fraction of (runaway) massive stars have masses exceeding
$60$-$70\, \rm M_{\odot}$ and are predicted to evolve as Luminous-Blue-Variable
and Wolf-Rayet stars before ending their lives as core-collapse supernovae. Our
2D axisymmetric hydrodynamical simulations explore how a fast wind ($2000\, \rm
km\, \rm s^{-1}$) and high mass-loss rate ($10^{-5}\, \rm M_{\odot}\, \rm
yr^{-1}$) can impact the morphology of the circumstellar medium. It is shaped
as 100 pc-scale wind nebula which can be pierced by the driving star when it
supersonically moves with velocity $20$-$40\, \rm km\, \rm s^{-1}$ through the
interstellar medium (ISM) in the Galactic plane. The motion of such runaway
stars displaces the position of the supernova explosion out of their bow shock
nebula, imposing asymmetries to the eventual shock wave expansion and
engendering Cygnus-loop-like supernova remnants. We conclude that the size (up
to more than $200\, \rm pc$) of the filamentary wind cavity in which the
chemically enriched supernova ejecta expand, mixing efficiently the wind and
ISM materials by at least $10\%$ in number density, can be used as a tracer of
the runaway nature of the very massive progenitors of such $0.1\, \rm Myr$ old
remnants. Our results motivate further observational campaigns devoted to the
bow shock of the very massive stars BD+43 3654 and to the close surroundings of
the synchrotron-emitting Wolf-Rayet shell G2.4+1.4.
|
Sustained formation of progenitor globular clusters in a giant
elliptical galaxy | Globular clusters (GCs) are thought to be ancient relics from the early
formative phase of galaxies, although their physical origin remains uncertain.
GCs are most numerous around massive elliptical galaxies, where they can
exhibit a broad colour dispersion, suggesting a wide metallicity spread. Here,
we show that many thousands of compact and massive (~5$\times$10$^{\rm
3}-$3$\times$ 10$^{\rm 6} M_{\odot}$) star clusters have formed at an
approximately steady rate over, at least, the past ~1Gyr around NGC 1275, the
central giant elliptical galaxy of the Perseus cluster. Beyond ~1Gyr, these
star clusters are indistinguishable in broadband optical colours from the more
numerous GCs. Their number distribution exhibits a similar dependence with
luminosity and mass as the GCs, whereas their spatial distribution resembles a
filamentary network of multiphase gas associated with cooling of the
intracluster gas. The sustained formation of these star clusters demonstrates
that progenitor GCs can form over cosmic history from cooled intracluster gas,
thus contributing to both the large number and broad colour dispersion$-$owing
to an age spread, in addition to a spread in metallicity$-$of GCs in massive
elliptical galaxies. The progenitor GCs have minimal masses well below the
maximal masses of Galactic open star clusters, affirming a common formation
mechanism for star clusters over all mass scales irrespective of their
formative pathways.
|
Secular Evolution and Structural Properties of Stellar Bars in Galaxies | I present results from the modeling of stellar bars in nearly 300 barred
galaxies in the local universe through parametric multi-component multi-band
image fitting. The surface brightness radial profile of bars is described using
a Sersic function, and parameters such as bar effective radius, ellipticity,
boxiness, length and mass, and bar-to-total luminosity and mass ratios, are
determined, which is unprecedented for a sample of this size. The properties of
bars in galaxies with classical bulges and pseudo-bulges are compared. For a
fixed bar-to-total mass ratio, pseudo-bulges are on average significantly less
massive than classical bulges, indicating that, if pseudo-bulges are formed
through bars, further processes are necessary to build a classical bulge. I
find a correlation between bar ellipticity and boxiness, and define a new
parameter as the product of these two quantities. I also find correlations
between this product and normalised bar size, between the sizes of bars and
bulges, and between normalised bar size and bulge-to-total ratio. Bars with
different ellipticities follow parallel lines in the latter two correlations.
These correlations can arise if, starting off with different normalised sizes
and ellipticities, bars grow longer and stronger with dynamical age, as a
result of angular momentum exchange from the inner to the outer parts of
galaxies, consistent with previous theoretical predictions. A plausible
consequence is that bar pattern speeds should become lower with bar dynamical
age, and towards galaxies with more prominent bulges.
|
Redistribution of Stars and Gas in the Star Formation Deserts of Barred
Galaxies | Bars strongly influence the distribution of gas and stars within the central
regions of their host galaxies. This is particularly pronounced in the star
formation desert (SFD) which is defined as two symmetrical regions either side
of the bar that show a deficit in young stars. Previous studies proposed that,
if star formation is truncated because of the influence of the bar, then the
age distribution of stars within the SFD could be used to determine the epoch
of bar formation. To test this, we study the properties of SFDs in 6 galaxies
from zoom-in cosmological re-simulations. Age maps reveal old regions on both
sides of the bars, with a lack of stars younger than 10 Myr, confirming the SFD
phenomenon. Local star formation is truncated in the SFDs because after the bar
forms, gas in these regions is removed on 1 Gyr timescales. However, the
overall age distribution of stars in the SFD does not show a sharp truncation
after bar formation but rather a gradual downturn in comparison to that of the
bar. This more subtle signature may still give information on bar formation
epochs in observed galaxies, but the interpretation will be more difficult than
originally hoped. The gradual drop in the SFD age distribution, instead of a
truncation, is due to radial migration of stars born in the disk. The SFD is
thus one of the only regions where an uncontaminated sample of stars only
affected by radial migration can be studied.
|
How Do Star-Forming Galaxies at Z>3 Assemble Their Masses? | We investigate how star-forming galaxies typically assemble their masses at
high redshift. Using the deep multi-wavelength coverage of the GOODS dataset,
we measure stellar mass of a large sample of star-forming galaxies at z~4 and
5, and make a robust determination of stellar mass function (SMF). We report a
broad correlation between stellar mass and UV luminosity, such that more
UV-luminous galaxies are, on average, more massive. However, the correlation
has a substantial intrinsic scatter evidenced by a non-negligible number of
UV-faint but massive galaxies. Furthermore, the low-mass end of the SMF does
not rise as steeply as the UV luminosity function (alpha_UVLF} -(1.7-1.8),
alpha_SMF -(1.3-1.4)) of the same galaxies. In a smooth formation scenario
where star formation (SF) is sustained at the observed rates for a long time,
these galaxies would have accumulated more mass (by a factor of ~3) than
observed and therefore the SMF would mirror more closely that of the UVLF. The
relatively shallow slope of the SMF is due to the fact that many of the
UV-selected galaxies are not massive enough, and therefore are too faint in
their rest-frame optical bands, to be detected in the current observations. Our
results favor a episodic formation history in which SF in low-mass galaxies
vary significantly over time, a scenario favored by galaxy clustering. Our
findings for the UV-faint galaxies are in contrast with those found for more
UV-luminous galaxies, which exhibit tighter SFR-M_star correlations. The
discrepancy may suggest that galaxies at different luminosities may have
different evolutionary paths.
|
Spectroscopic surveys of massive AGB and super-AGB stars | It is now about 30 years ago that photometric and spectroscopic surveys of
asymptotic giant branch (AGB) stars in the Magellanic Clouds (MCs) uncovered
the first examples of truly massive (> 3-4 M_s) O-rich AGB stars experiencing
hot bottom burning (HBB). Massive (Li-rich) HBB AGB stars were later identified
in our own Galaxy and they pertain to the Galactic population of obscured OH/IR
stars. High-resolution optical spectroscopic surveys have revealed the massive
Galactic AGB stars to be strongly enriched in Rb compared to other nearby
s-process elements like Zr, confirming that Ne22 is the dominant neutron source
in these stars. Similar surveys of OH/IR stars in the MCs disclosed their
Rb-rich low-metallicity counterparts, showing that these stars are usually
brighter (because of HBB flux excess) than the standard adopted luminosity
limit for AGB stars (Mbol~-7.1) and that they might have stellar masses of at
least ~6-7 M_s. The chemical composition and photometric variability are
efficient separating the massive AGB stars from massive red supergiants (RSG)
but the main difficulty is to distinguish between massive AGB and super-AGB
stars because the present theoretical nucleosynthesis models predict both stars
to be chemically identical. Here I review the available multiwavelength (from
the optical to the far-IR) observations on massive AGB and super-AGB stars as
well as the current caveats and limitations in our undestanding of these stars.
Finally, I underline the expected observations on massive AGB and super-AGB
stars from on-going massive surveys like Gaia and SDSS-IV/APOGEE-2 and future
facilities such as the James Webb Space Telescope.
|
The bounce universe history from unimodular $F(R)$ gravity | In this paper we investigate how to realize various quite well known
cosmological bouncing models in the context of the recently developed
unimodular $F(R)$ gravity. Particularly, we shall study the matter bounce
scenario, the singular bounce, the superbounce and a symmetric bounce scenario.
We present the behavior of the Hubble radius for each of the bouncing models we
shall take into account and we investigate which era of the bouncing model is
responsible for the cosmological perturbations. As we shall demonstrate, the
various bouncing models do not behave in the same way, so the cosmological
perturbations for each model may correspond to a different era, in comparison
to other models. Also we present which unimodular $F(R)$ gravity realizes each
model. We also show that Newton's law is not modified in the unimodular $F(R)$
gravity, which also is proven to be a ghost-free theory, and in addition we
discuss the matter stability issue. Finally, we demonstrate how it is possible
to solve a cosmological constant problem in the context of unimodular $F(R)$
gravity.
|
Lyman Alpha Blobs as an Observational Signature of Cold Accretion
Streams into Galaxies | Recent hydrodynamic simulations of galaxy formation reveal streams of cold (T
~ 1e4 K) gas flowing into the centers of dark matter halos as massive as
1e12-1e13.5 M_sun at redshifts z~1-3. In this paper we show that if > 20% of
the gravitational binding energy of the gas is radiated away, then the
simulated cold flows are spatially extended Lyman Alpha (Lya) sources with
luminosities, Lya line widths, and number densities that are comparable to
those of observed Lya blobs. Furthermore, the filamentary structure of the cold
flows can explain the wide range of observed Lya blob morphologies. Since the
most massive halos form in dense environments, the association of Lya blobs
with overdense regions arise naturally. We argue that Lya blobs - even those
which are clearly associated with starburst galaxies or quasars - provide
direct observational support for the cold accretion mode of galaxies. We
discuss various testable predictions of this association.
|
Dark sectors of the Universe: A Euclid survey approach | In this paper we study the consequences of relaxing the hypothesis of the
pressureless nature of the dark matter component when determining constraints
on dark energy. To this aim we consider simple generalized dark matter models
with constant equation of state parameter. We find that present-day
low-redshift probes (type-Ia supernovae and baryonic acoustic oscillations)
lead to a complete degeneracy between the dark energy and the dark matter
sectors. However, adding the cosmic microwave background (CMB) high-redshift
probe restores constraints similar to those on the standard $\Lambda$CDM model.
We then examine the anticipated constraints from the galaxy clustering probe of
the future Euclid survey on the same class of models, using a Fisher forecast
estimation. We show that the Euclid survey allows us to break the degeneracy
between the dark sectors, although the constraints on dark energy are much
weaker than with standard dark matter. The use of CMB in combination allows us
to restore the high precision on the dark energy sector constraints.
|
Is water ice an efficient facilitator for dust coagulation? | Beyond the snow line of protoplanetary discs and inside the dense core of
molecular clouds, the temperature of gas is low enough for water vapour to
condense into amorphous ices on the surface of preexisting refractory dust
particles. Recent numerical simulations and laboratory experiments suggest that
condensation of the vapour promotes dust coagulation in such a cold region.
However, in the numerical simulations, cohesion of refractory materials is
often underestimated, while in the laboratory experiments, water vapour
collides with surfaces at more frequent intervals compared to the real
conditions. Therefore, to re-examine the role of water ice in dust coagulation,
we carry out systematic investigation of available data on coagulation of water
ice particles by making full use of appropriate theories in contact mechanics
and tribology. We find that the majority of experimental data are reasonably
well explained by lubrication theories, owing to the presence of a quasi-liquid
layer (QLL). Only exceptions are the results of dynamic collisions between
particles at low temperatures, which are, instead, consistent with the JKR
theory, because QLLs are too thin to dissipate their kinetic energies. By
considering the vacuum conditions in protoplanetary discs and molecular clouds,
the formation of amorphous water ice on the surface of refractory particles
does not necessarily aid their collisional growth as currently expected. While
crystallisation of water ice around but outside the snow line eases coagulation
of ice-coated particles, sublimation of water ice inside the snow line is
deemed to facilitate coagulation of bare refractory particles.
|
An effective description of dark matter and dark energy in the mildly
non-linear regime | In the next few years, we are going to probe the low-redshift universe with
unprecedented accuracy. Among the various fruits that this will bear, it will
greatly improve our knowledge of the dynamics of dark energy, though for this
there is a strong theoretical preference for a cosmological constant. We assume
that dark energy is described by the so-called Effective Field Theory of Dark
Energy, which assumes that dark energy is the Goldstone boson of time
translations. Such a formalism makes it easy to ensure that our signatures are
consistent with well-established principles of physics. Since most of the
information resides at high wavenumbers, it is important to be able to make
predictions at the highest wavenumber that is possible. The Effective Field
Theory of Large-Scale Structure (EFTofLSS) is a theoretical framework that has
allowed us to make accurate predictions in the mildly non-linear regime. In
this paper, we derive the non-linear equations that extend the EFTofLSS to
include the effect of dark energy both on the matter fields and on the biased
tracers. For the specific case of clustering quintessence, we then
perturbatively solve to cubic order the resulting non-linear equations and
construct the one-loop power spectrum of the total density contrast.
|
Semi-implicit anisotropic cosmic ray transport on an unstructured moving
mesh | In the interstellar medium of galaxies and the intracluster gas of galaxy
clusters, the charged particles making up cosmic rays are moving almost
exclusively along (but not across) magnetic field lines. The resulting
anisotropic transport of cosmic rays in the form of diffusion or streaming not
only affects the gas dynamics but also rearranges the magnetic fields
themselves. The coupled dynamics of magnetic fields and cosmic rays can thus
impact the formation and evolution of galaxies and the thermal evolution of
galaxy clusters in critical ways. Numerically studying these effects requires
solvers for anisotropic diffusion that are accurate, efficient, and robust,
requirements that have proven difficult to satisfy in practice. Here, we
present an anisotropic diffusion solver on an unstructured moving mesh that is
conservative, does not violate the entropy condition, allows for semi-implicit
time integration with individual timesteps, and only requires solving a single
linear system of equations per timestep. We apply our new scheme to a large
number of test problems and show that it works as well or better than previous
implementations. Finally, we demonstrate for a numerically demanding simulation
of the formation of an isolated disk galaxy that our local time-stepping scheme
reproduces the results obtained with global time-stepping at a fraction of the
computational cost.
|
The detection of a massive chain of dark HI clouds in the GAMA G23 Field | We report on the detection of a large, extended HI cloud complex in the GAMA
G23 field, located at a redshift of $z\,\sim\,0.03$, observed as part of the
MeerHOGS campaign (a pilot survey to explore the mosaicing capabilities of
MeerKAT). The cloud complex, with a total mass of $10^{10.0}\,M_\odot$, lies in
proximity to a large galaxy group with $M_\mathrm{dyn}\sim10^{13.5}\,M_\odot$.
We identify seven HI peak concentrations, interconnected as a tenuous 'chain'
structure, extending $\sim 400\,\mathrm{kpc}$ from east-to-west, with the
largest (central) concentration containing $10{^{9.7}}\,M_\odot$ in HI gas
distributed across $50\,\mathrm{kpc}$. The main source is not detected in
ultra-violet, optical or infrared imaging. The implied gas mass-to-light
($M_\mathrm{HI}$/$L_\mathrm{r}$) is extreme ($>$1000) even in comparison to
other 'dark clouds'. The complex has very little kinematic structure
($110\,\mathrm{km}\,\mathrm{s}^{-1}$), making it difficult to identify cloud
rotation. Assuming pressure support, the total mass of the central
concentration is $>10^{10.2}\,M_\odot$, while a lower limit to the dynamical
mass in the case of full rotational support is $10^{10.4}\,M_\odot$. If the
central concentration is a stable structure, it has to contain some amount of
unseen matter, but potentially less than is observed for a typical galaxy. It
is, however, not clear whether the structure has any gravitationally stable
concentrations. We report a faint UV--optical--infrared source in proximity to
one of the smaller concentrations in the gas complex, leading to a possible
stellar association. The system nature and origins is enigmatic, potentially
being the result of an interaction with or within the galaxy group it appears
to be associated with.
|
On the chemical ladder of esters. Detection and formation of ethyl
formate in the W51 e2 hot molecular core | The detection of organic molecules with increasing complexity and potential
biological relevance is opening the possibility to understand the formation of
the building blocks of life in the interstellar medium. One of the families of
molecules with astrobiological interest are the esters, whose simplest member,
methyl formate, is rather abundant in star-forming regions. The next step in
the chemical complexity of esters is ethyl formate, C$_2$H$_5$OCHO. Only two
detections of this species have been reported so far, which strongly limits our
understanding of how complex molecules are formed in the interstellar medium.
We have searched for ethyl formate towards the W51 e2 hot molecular core, one
of the most chemically rich sources in the Galaxy and one of the most promising
regions to study prebiotic chemistry, especially after the recent discovery of
the P$-$O bond, key in the formation of DNA. We have analyzed a spectral line
survey towards the W51 e2 hot molecular core, which covers 44 GHz in the 1, 2
and 3 mm bands, carried out with the IRAM 30m telescope. We report the
detection of the trans and gauche conformers of ethyl formate. A Local
Thermodynamic Equilibrium analysis indicates that the excitation temperature is
78$\pm$10 K and that the two conformers have similar source-averaged column
densities of (2.0$\pm$0.3)$\times$10$^{16}$ cm$^{-2}$ and an abundance of
$\sim$10$^{-8}$. We compare the observed molecular abundances of ethyl formate
with different competing chemical models based on grain surface and gas-phase
chemistry. We propose that grain-surface chemistry may have a dominant role in
the formation of ethyl formate (and other complex organic molecules) in hot
molecular cores, rather than reactions in the gas phase.
|
The Intergalactic Stellar Population from Mergers of Elliptical Galaxies
with Dark Matter halos | We present simulations of dry-merger encounters between pairs of elliptical
galaxies with dark matter halos. The aim of these simulations is to study the
intergalactic stellar populations produced in both parabolic and hyperbolic
encounters. We model progenitor galaxies with total-to-luminous mass ratios
M_T/M_L 3 and 11. The initial mass of the colliding galaxies are chosen so that
M_1/M_2 and 10. The model galaxies are populated by particles representing
stars, as in Stanghellini et al. (2006), and dark matter. Merger remnants
resulting from these encounters display a population of unbounded particles,
both dark and luminous. The number of particles becoming unbounded depends on
orbital configuration, with hyperbolic encounters producing a larger luminous
intracluster population than parabolic encounters. Furthermore, in simulations
with identical orbital parameters, a lower M_T/M_L of the colliding galaxies
produces a larger fraction of unbounded luminous particles. For each modeled
collision, the fraction of unbounded to initial stellar mass is the same in all
mass-bins considered, similarly to what we found previously by modeling
encounters of galaxies without dark halos. The fraction of intergalactic to
total luminosity resulting from our simulations is ~ 4% and ~ 6% for
dark-to-bright mass rations of 10 and 2 respectively. These unbounded-to-total
luminous fractions are down from 17 % that we had previously found in the case
of no dark halos. Our results are in broad agreement with intergalactic light
observed in groups of galaxies, while the results of our previous models
without dark halos better encompass observed intracluster populations. We
suggest a possible formation scenario of intergalactic stars.
|
Colours of Bulges and Discs within Galaxy Clusters and the Signature of
Disc Fading on Infall | The origins of the bulge and disc components of galaxies are of primary
importance to understanding galaxy formation. Here bulge-disc decomposition is
performed simultaneously in B- and R-bands for 922 bright galaxies in 8 nearby
(z < 0.06) clusters with deep redshift coverage using photometry from the NOAO
Fundamental Plane Survey. The total galaxy colours follow a universal
colour-magnitude relation (CMR). The discs of L_* galaxies are 0.24 magnitudes
bluer in $B-R$ than bulges. Bulges have a significant CMR slope while the CMR
slope of discs is flat. Thus the slope of the CMR of the total light is driven
primarily (60%) by the bulge-CMR, and to a lesser extent (40%) by the change in
the bulge-to-total ratio as a function of magnitude. The colours of the bulge
and disc components do not depend on the bulge-to-total ratio, for galaxies
with bulge-to-total ratios greater than 0.2. While the colours of the bulge
components do not depend significantly on environment, the median colours of
discs vary significantly, with discs in the cluster centre redder by 0.10
magnitudes than those at the virial radius. Thus while star formation in bulges
appears to be regulated primarily by mass-dependent, and hence presumably
internal, processes, that of discs is affected by the cluster environment.
|
The TeV binary HESS J0632+057 in the low and high X-ray state | We report on a 40ks Chandra observation of the TeV emitting high mass X-ray
binary HESS J0632+057 performed in February 2011 during a high-state of X-ray
and TeV activity. We have used the ACIS-S camera in Continuos Clocking mode to
search for a possible X-ray pulsar in this system. Furthermore, we compare the
emission of the source during this high state, with its X-ray properties during
a low state of emission, caught by a 47ks XMM-Newton observation on September
2007. We did not find any periodic or quasi-periodic signal in any of the two
observations. We derived an average pulsed fraction 3sigma upper limit for the
presence of a periodic signal of ~35% and 25% during the low and high emission
state, respectively (although this limit is strongly dependent on the frequency
and the energy band). Using the best X-ray spectra derived to date for HESS
J0632+057, we found evidence for a significant spectral change between the low
and high X-ray emission states, with the absorption value and the photon index
varying between Nh ~ 2.1-4.3x10^{21} cm^{-2} and Gamma ~ 1.18-1.61. At variance
with what observed in other TeV binaries, it seems that in this source the
higher the flux the softer the X-ray spectrum.
|
This is not the feedback you have been looking for: nearby optical AGN
rarely drive kpc-scale cold-gas outflows | We study the interstellar Na I $\lambda \lambda 5890, 5895$ (Na D)
absorption-line doublet in a nearly-complete sample of $\sim$9900 nearby
Seyfert 2 galaxies, in order to quantify the significance of optical AGN
activity in driving kpc-scale outflows that can quench star formation.
Comparison to a carefully matched sample of $\sim$44,000 control objects
indicates that the Seyfert and control population have similar Na D detection
rates ($\sim 5-6%$). Only 53 Seyferts (or 0.5% of the population) are found to
potentially display galactic-scale winds, compared to 0.8% of the control
galaxies. While nearly a third of the Na D outflows observed in our Seyfert 2
galaxies occur around the brightest AGN, both radio and infrared data indicate
that star formation could play the dominant role in driving cold-gas outflows
in an even higher fraction of the Na D-outflowing Seyfert 2s. Our results
indicate that galactic-scale outflows at low redshift are no more frequent in
Seyferts than they are in their non-active counterparts, that optical AGN are
not significant contributors to the quenching of star formation in the nearby
Universe, and that star-formation may actually be the principal driver of
outflows even in systems that do host an AGN.
|
Gas and dark matter in the Sculptor group: NGC 55 | We present new, sensitive HI observations of the Sculptor group galaxy NGC 55
with the Australia Telescope Compact Array. We achieve a 5 sigma HI column
density sensitivity of 10^19 cm^-2 over a spectral channel width of 8 km/s for
emission filling the 158" x 84" synthesised beam. Our observations reveal for
the first time the full extent of the HI disc of NGC 55 at this sensitivity and
at a moderately high spatial resolution of about 1 kpc.
The HI disc of NGC 55 appears to be distorted on all scales. There is a
strong east-west asymmetry in the column density distribution along the major
axis, suggesting that the disc is under the influence of ram-pressure forces.
We also find evidence of streaming motions of the gas along the bar of NGC 55.
The fitting of tilted rings to the velocity field reveals a strong warping of
the outer gas disc which could be the result of tidal interaction with either
NGC 300 or a smaller satellite galaxy. Finally, we find a large number of
distinct clumps and spurs across the entire disc, indicating that internal or
external processes, such as satellite accretion or gas outflows, have stirred
up the gas disc.
We also detect several isolated HI clouds within about 20 kpc projected
distance from NGC 55. Their dynamical properties and apparent concentration
around NGC 55 suggest that most of the clouds are forming a circum-galactic
population similar to the high-velocity clouds of the Milky Way and M31,
although two of the clouds could be foreground objects and part of the
Magellanic Stream. While it is difficult to determine the origin of these
clouds, our data seem to favour either tidal stripping or gas outflows as the
source of the gas.
|
HI as a Probe of the Large Scale Structure in the Post-Reionization
Universe: Visibility Correlations and Prospects for Detection | Simulated maps of the HI distribution in the post-reionization era are used
to study the prospects for detection with existing and upcoming radio
telescopes. We consider detection in the redshifted radiation from the
hyperfine transition with a rest frame frequency of 1420 MHz. Possibility of a
statistical detection using visibility correlations is discussed. We show that
the MWA (Murchison Widefield Array) and the GMRT (Giant
Meterwave Radio Telescope) can potentially detect signal from the HI
distribution at high redshifts. MWA can detect visibility correlations at large
angular scales at all redshifts accessible to it in the post-reionization era.
The GMRT can detect visibility correlations at lower redshifts, specifically
there is a strong case for a survey at z = 1.3. We also discuss prospects for
direct detection of rare peaks in the HI distribution using the GMRT. We show
that direct detection should be possible with integration time that is
comparable to, or even less than, the time required for a statistical
detection. Specifically, it is possible to make a statistical detection of the
HI distribution by measuring the visibility correlation, and, direct detection
of rare peaks in the HI distribution using the GMRT in less than 1000 hours of
observations.
|
Primordial Black Holes in Higgs-$R^2$ Inflation as a Whole Dark Matter | Primordial black holes are produced in a minimal UV extension to the Higgs
inflation with an included $R^2$ term. We show that for parameters consistent
with Standard Model measurements and Planck observation results lead to $M_{\rm
PBH} \in (10^{-16}, 10^{-15}) M_\odot$ primordial black holes with significant
abundance, which may consist the majority of dark matter.
|
Isolated Massive Star Formation in G28.20-0.05 | We report high-resolution 1.3mm continuum and molecular line observations of
the massive protostar G28.20-0.05 with ALMA. The continuum image reveals a
ring-like structure with 2,000 au radius, similar to morphology seen in
archival 1.3cm VLA observations. Based on its spectral index and associated
H30$\alpha$ emission, this structure mainly traces ionized gas. However, there
is evidence for $\sim$30 M$\odot$ of dusty gas near the main mm continuum peak
on one side of the ring, as well as in adjacent regions within 3,000 au. A
virial analysis on scales of $\sim$2,000 au from hot core line emission yields
a dynamical mass of $\sim$80M$\odot$. A strong velocity gradient in the
H30$\alpha$ emission is evidence for a rotating, ionized disk wind, which
drives a larger-scale molecular outflow. An infrared SED analysis indicates a
current protostellar mass of m$_{star}\sim$24 M$\odot$ forming from a core with
initial mass $M_c\sim400\:M_\odot$ in a clump with mass surface density of
$\Sigma_{\rm cl}\sim 3\:{\rm g\:cm}^{-2}$. Thus the SED and other properties of
the system can be understood in the context of core accretion models.
Structure-finding analysis on the larger-scale continuum image indicates
G28.20-0.05 is forming in a relatively isolated environment, with no other
concentrated sources, i.e., protostellar cores, above $\sim$ 1 M$\odot$ found
from $\sim$0.1 to 0.4 pc around the source. This implies that a massive star is
able to form in relative isolation and the dearth of other protostellar
companions within the $\sim$1 pc environs is a strong constraint on massive
star formation theories that predict the presence of a surrounding
protocluster.
|
Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas | While the CO(1-0) transition is often used to deduce the total molecular
hydrogen in galaxies, it is challenging to detect in low metallicity galaxies,
in spite of the star formation taking place. In contrast, the [CII] 158 micron
line is relatively bright, highlighting a potentially important reservoir of H2
that is not traced by CO(1-0), but residing in the C+ - emitting regions. We
explore a method to quantify the total H2 mass (MH2) in galaxies and learn what
parameters control the CO-dark gas reservoir. We present Cloudy grids of
density, radiation field and metallicity in terms of observed quantities, such
as [OI], [CI], CO(1-0), [CII], total infrared luminosity and the total MH2 and
provide recipes based on these models to derive total MH2 mass estimates from
observations. The models are applied to the Herschel Dwarf Galaxy Survey,
extracting the total MH2 for each galaxy which is compared to the H2 determined
from the observed CO(1-0) line. While the H2 traced by CO(1-0) can be
negligible, the [CII] 158 micron line can trace the total H2. 70% to 100% of
the total H2 mass is not traced by CO(1-0) in the dwarf galaxies, but is
well-traced by [CII] 158 micron line. The CO-dark gas mass fraction correlates
with the observed L[CII]/LCO(1-0) ratio. A conversion factor for [CII]
luminosity to total H2 and a new CO-to-total-MH2 conversion factor, as a
function of metallicity, is presented. A recipe is provided to quantify the
total mass of H2 in galaxies, taking into account the CO and [CII]
observations. Accounting for this CO-dark H2 gas, we find that the star forming
dwarf galaxies now fall on the Schmidt-Kennicutt relation. Their star-forming
efficiency is rather normal, since the reservoir from which they form stars is
now more massive when introducing the [CII] measures of the total H2, compared
to the little amount of H2 in the CO-emitting region.
|
LARgE Survey -- III. Environments of Ultra-Massive Passive Galaxies at
Cosmic Noon: BCG progenitors growing through mergers | We study the environments of a sample of 61 extremely rare z~1.6
Ultra-Massive Passively Evolving Galaxies (UMPEGs: stellar masses M_stars
>10^11.5 M_sun) which -- based on clustering analysis presented in Cheema et
al. (2020) -- appear to be associated with very massive (M_halo ~ 10^14.1 h^-1
M_sun) dark matter halos that are likely to be the progenitors of z~0 massive
(Coma- and Virgo-like) galaxy clusters. We find that UMPEGs on average have
fewer than one satellite galaxy with mass ratio M_sat : M_UMPEG >~ 1:5 (i.e.,
M_sat >~ 10^10.8 M_sun) within 0.5 Mpc; the large mass gap that we observe
between the typical UMPEG and its most massive satellite implies that the z~1.6
UMPEGs assembled through major mergers. Using observed satellite counts with
merger timescales from the literature, we estimate the growth rate due to
mergers with mass ratio of >~ 1:4 to be ~13% Gyr^-1 (with a ~2x systematic
uncertainty). This relatively low growth rate is unlikely to significantly
affect the shape of the massive end of the stellar mass function, whose
evolution must instead be driven by the quenching of new cohorts of
ultra-massive star-forming galaxies. However, this growth rate is high enough
that, if sustained to z~0, the typical z~1.6 M_UMPEG=10^11.6 M_sun UMPEG can
grow into a M_stars~10^12 M_sun brightest cluster galaxy (BCG) of a present-day
massive galaxy cluster. Our observations favour a scenario in which our UMPEGs
are main-branch progenitors of some of the present-day BCGs that have first
assembled through major mergers at high redshifts and grown further through
(likely minor) merging at later times.
|
Assessing the stellar population and the environment of an HII region on
the far side of the Galaxy | We have investigated the stellar and interstellar content of the distant star
formation region IRAS 17591-2228 (WISE HII region GAL 007.47+0.06). It is
associated to a water maser, whose parallax distance is d=20.4^{+2.8} {-2.2}
kpc, supported by independent measurements of proper motion and radial
velocity. It is projected in the same direction as an extremely red (J-Ks ~ 6
mag) group of stars, and a shell of mid-infrared emission. We qualify the group
of stars as a cluster candidate, VVV CL177. Its radius spans between 0.45' and
1' and contains at least two young stellar objects with an extreme extinction
near Av ~ 40 mag. Yet more analysis will be required to determine is it is a
real single cluster associated with the water maser. The 13CO emissions at the
radial velocity of the maser corresponds to the mid-infrared emission.
|
Carbon-Enhanced Metal-Poor Stars: Relics from the Dark Ages | We use detailed nucleosynthesis calculations and a realistic prescription for
the environment of the first stars to explore the first episodes of chemical
enrichment that occurred during the dark ages. Based on these calculations, we
propose a novel explanation for the increased prevalence of carbon-enhanced
metal-poor (CEMP) stars with decreasing Fe abundance: The observed chemistry
for the most metal-poor Galactic halo stars is the result of an intimate link
between the explosions of the first stars and their host minihalo's ability to
retain its gas. Specifically, high-energy supernovae produce a near solar ratio
of C/Fe, but are effective in evacuating the gas from their host minihalo,
thereby suppressing the formation of a second generation of stars. On the other
hand, minihalos that host low-energy supernovae are able to retain their gas
and form a second stellar generation but, as a result, the second stars are
born with a supersolar ratio of C/Fe. Our models are able to accurately
reproduce the observed distributions of [C/Fe] and [Fe/H], as well as the
fraction of CEMP stars relative to non-CEMP stars as a function of [Fe/H]
without any free parameters. We propose that the present lack of chemical
evidence for very massive stars (>140 Msun), that ended their lives as a highly
energetic pair-instability supernova, does not imply that such stars were rare
or did not exist; the chemical products of these very massive first stars may
have escaped from their host minihalo, and were never incorporated into
subsequent generations of stars. Finally, our models suggest that the most
Fe-poor stars currently known may have seen the enrichment from a small
multiple of metal-free stars, and need not have been exclusively enriched by a
solitary first star. These calculations support the idea that some of the
surviving dwarf satellite galaxies of the Milky Way are relics of the first
galaxies.
|
Low-Redshift Ly-alpha Selected Galaxies from GALEX Spectroscopy: A
Comparison with Both UV-Continuum Selected Galaxies and High-Redshift
Ly-alpha Emitters | We construct a sample of low-redshift Ly-alpha emission-line selected sources
from GALEX grism spectroscopy of nine deep fields to study the role of Ly-alpha
emission in galaxy populations with cosmic time. Our final sample consists of
119 (141) sources selected in the redshift interval z=0.195-0.44 (z=0.65-1.25)
from the FUV (NUV) channel. We classify the Ly-alpha sources as AGNs if
high-ionization emission lines are present in their UV spectra and as possible
star-forming galaxies otherwise. We classify additional sources as AGNs using
line widths for our Ly-alpha emitter (LAE) analysis. These classifications are
broadly supported by comparisons with X-ray and optical spectroscopic
observations. Defining the GALEX LAE sample in the same way as high-redshift
LAE samples, we show that LAEs constitute only about 5% of NUV-continuum
selected galaxies at z~0.3. We also show that they are less common at z~0.3
than they are at z~3. We find that the z~0.3 optically-confirmed Ly-alpha
galaxies lie below the metallicity-luminosity relation of the z~0.3
NUV-continuum selected galaxies but have similar H-alpha velocity widths at
similar luminosities, suggesting that they also lie below the metallicity-mass
relation of the NUV-continuum selected galaxies. We show that, on average, the
Ly-alpha galaxies have bluer colors, lower extinctions as measured from the
Balmer line ratios, and more compact morphologies than the NUV-continuum
selected galaxies. Finally, we confirm that the z~2 Lyman break galaxies (LBGs)
have relatively low metallicities for their luminosities, and we find that they
lie in the same metallicity range as the z~0.3 Ly-alpha galaxies.
|
Effective $f(R)$ actions for modified Loop Quantum Cosmologies via order
reduction | General Relativity is an extremely successful theory, at least for weak
gravitational fields, however, it breaks down at very high energies, such as in
correspondence of the initial singularity. Quantum Gravity is expected to
provide more physical insights concerning this open question. Indeed, one
alternative scenario to the Big Bang, that manages to completely avoid the
singularity, is offered by Loop Quantum Cosmology (LQC), which predicts that
the Universe undergoes a collapse to an expansion through a bounce. In this
work, we use metric $f(R)$ gravity to reproduce the modified Friedmann
equations which have been obtained in the context of modified loop quantum
cosmologies. To achieve this, we apply an order reduction method to the $f(R)$
field equations, and obtain covariant effective actions that lead to a bounce,
for specific models of modified LQC, considering matter as a scalar field.
|
Tracking Down the Source Population Responsible for the Unresolved
Cosmic 6-8 keV Background | Using the 4 Ms Chandra Deep Field-South (CDF-S) survey, we have identified a
sample of 6845 X-ray undetected galaxies that dominates the unresolved ~ 20-25%
of the 6-8 keV cosmic X-ray background (XRB). This sample was constructed by
applying mass and color cuts to sources from a parent catalog based on
GOODS-South HST z-band imaging of the central 6'-radius area of the 4 Ms CDF-S.
The stacked 6-8 keV detection is significant at the 3.9 sigma level, but the
stacked emission was not detected in the 4-6 keV band which indicates the
existence of an underlying population of highly obscured active galactic nuclei
(AGNs). Further examinations of these 6845 galaxies indicate that the galaxies
on the top of the blue cloud and with redshifts of 1 < z < 3, magnitudes of 25
< z_850 < 28, and stellar masses of 2E8 < M_star/M_sun < 2E9 make the majority
contributions to the unresolved 6-8 keV XRB. Such a population is seemingly
surprising given that the majority of the X-ray detected AGNs reside in massive
(> ~1E10 M_sun) galaxies. We discuss constraints upon this underlying AGN
population, supporting evidence for relatively low-mass galaxies hosting highly
obscured AGNs, and prospects for further boosting the stacked signal.
|
Reclassification of the nearest quasar pair candidate: SDSS J15244+3032
- RXS J15244+3032 | We present optical spectroscopy of the nearest quasar pair listed in the 13th
edition of the Veron-Cetty & Veron catalogue, i.e. the two quasars SDSS
J15244+3032 and RXS J15244+3032 (redshift z~0.27, angular separation ~7 arcsec,
and line-of-sight velocity difference ~1900 km/s). This system would be an
optimal candidate to investigate the mutual interaction of the host galaxies
with ground based optical imaging and spectroscopy. However, new optical data
demonstrate that RXS J15244+3032 is indeed a star of spectral type G.
This paper includes data gathered with the Asiago 1.82m telescope (Cima Ekar
Observatory, Asiago, Italy).
|
On the use of black hole binaries as probes of local dark energy
properties | Accretion of dark energy onto black holes will take place when dark energy is
not a cosmological constant. It has been proposed that the time evolution of
the mass of the black holes in binary systems due to dark energy accretion
could be detectable by gravitational radiation. This would make it possible to
use observations of black hole binaries to measure local dark energy
properties, e.g., to determine the sign of 1+w where w is the dark energy
equation of state. In this Letter we show that such measurements are unfeasible
due to the low accretion rates.
|
Constraint on energy-momentum squared gravity from neutron stars and its
cosmological implications | Deviations from the predictions of general relativity due to energy-momentum
squared gravity (EMSG) are expected to become pronounced in the high density
cores of neutron stars. We derive the hydrostatic equilibrium equations in EMSG
and solve them numerically to obtain the neutron star mass-radius relations for
four different realistic equations of state. We use the existing observational
measurements of the masses and radii of neutron stars to constrain the free
parameter, $\alpha ,$ that characterizes the coupling between matter and
spacetime in EMSG. We show that $-10^{-38}\,\mathrm{cm^{3}/erg}<\alpha
<+10^{-37}\,\mathrm{cm^{3}/erg}$. Under this constraint, we discuss what
contributions EMSG can provide to the physics of neutron stars, in particular,
their relevance to the so called \textit{hyperon puzzle} in neutron stars. We
also discuss how EMSG alters the dynamics of the early universe from the
predictions of the standard cosmological model. We show that EMSG leaves the
standard cosmology safely unaltered back to $t\sim 10^{-4}$ seconds at which
the energy density of the universe is $\sim 10^{34}\,\mathrm{erg\,cm^{-3}}$.
|
Exponentially growing bubbles around early super massive black holes | We addressed the so far unexplored issue of outflows induced by exponentially
growing power sources, focusing on early supermassive black holes (BHs). We
assumed that these objects grow to $10^9\;M_{\odot}$ by z=6 by
Eddington-limited accretion and convert 5% of their bolometric output into a
wind. We first considered the case of energy-driven and momentum-driven
outflows expanding in a region where the gas and total mass densities are
uniform and equal to the average values in the Universe at $z>6$. We derived
analytic solutions for the evolution of the outflow, finding that, for an
exponentially growing power with e-folding time $t_{Sal}$, the late time
expansion of the outflow radius is also exponential, with e-folding time of
$5t_{Sal}$ and $4t_{Sal}$ in the energy-driven and momentum-driven limit,
respectively.
We then considered energy-driven outflows produced by QSOs at the center of
early dark matter halos of different masses and powered by BHs growing from
different seeds. We followed the evolution of the source power and of the gas
and dark matter density profiles in the halos from the beginning of the
accretion until $z=6$. The final bubble radius and velocity do not depend on
the seed BH mass but are instead smaller for larger halo masses. At z=6, bubble
radii in the range 50-180 kpc and velocities in the range 400-1000 km s$^{-1}$
are expected for QSOs hosted by halos in the mass range
$3\times10^{11}-10^{13}\;M_{\odot}$.
By the time the QSO is observed, we found that the total thermal energy
injected within the bubble in the case of an energy-driven outflow is
$E_{th}\sim5 \times 10^{60}$ erg. This is in excellent agreement with the value
of $E_{th}=(6.2\pm 1.7)\times 10^{60}$ erg measured through the detection of
the thermal Sunyaev-Zeldovich effect around a large population of luminous QSOs
at lower redshift. [abridged]
|
Quantifying and controlling biases in dark matter halo concentration
estimates | We use bootstrapping to estimate the bias of concentration estimates on
N-body dark matter halos as a function of particle number. We find that
algorithms based on the maximum radial velocity and radial particle binning
tend to overestimate the concentration by 15%-20% for halos sampled with 200
particles and by 7% - 10% for halos sampled with 500 particles. To control this
bias at low particle numbers we propose a new algorithm that estimates halo
concentrations based on the integrated mass profile. The method uses the full
particle information without any binning, making it reliable in cases when low
numerical resolution becomes a limitation for other methods. This method
reduces the bias to less than 3% for halos sampled with 200-500 particles. The
velocity and density methods have to use halos with at least 4000 particles in
order to keep the biases down to the same low level. We also show that the
mass-concentration relationship could be shallower than expected once the
biases of the different concentration measurements are taken into account.
These results show that bootstrapping and the concentration estimates based on
the integrated mass profile are valuable tools to probe the internal structure
of dark matter halos in numerical simulations.
|
Extranatural Flux Inflation | We propose a new inflation scenario in flux compactification, where a zero
mode scalar field of extra components of the higher dimensional gauge field is
identified with an inflaton. The scalar field is a pseudo Nambu-Goldstone boson
of spontaneously broken translational symmetry in compactified spaces. The
inflaton potential is non-local and finite, which is protected against the
higher dimensional non-derivative local operators by quantum gravity
corrections thanks to the gauge symmetry in higher dimensions and the shift
symmetry originated from the translation in extra spaces. We give an explicit
inflation model in a six dimensional scalar QED, which is shown to be
consistent with Planck 2018 data.
|
The Formation of Kiloparsec-Scale HI Holes in Dwarf Galaxies | The origin of kpc-scale holes in the atomic hydrogen (H I) distributions of
some nearby dwarf irregular galaxies presents an intriguing problem. Star
formation histories (SFHs) derived from resolved stars give us the unique
opportunity to study past star forming events that may have helped shape the
currently visible H I distribution. Our sample of five nearby dwarf irregular
galaxies spans over an order of magnitude in both total H I mass and absolute
B-band magnitude and is at the low mass end of previously studied systems. We
use Very Large Array H I line data to estimate the energy required to create
the centrally dominant hole in each galaxy. We compare this energy estimate to
the past energy released by the underlying stellar populations computed from
SFHs derived from data taken with the Hubble Space Telescope. The inferred
integrated stellar energy released within the characteristic ages exceeds our
energy estimates for creating the holes in all cases, assuming expected
efficiencies. Therefore, it appears that stellar feedback provides sufficient
energy to produce the observed holes. However, we find no obvious signature of
single star forming events responsible for the observed structures when
comparing the global SFHs of each galaxy in our sample to each other or to
those of dwarf irregular galaxies reported in the literature. We also fail to
find evidence of a central star cluster in FUV or Halpha imaging. We conclude
that large H I holes are likely formed from multiple generations of star
formation and only under suitable interstellar medium conditions.
|
Constraints on primordial black holes as dark matter candidates from
capture by neutron stars | We investigate constraints on primordial black holes (PBHs) as dark matter
candidates that arise from their capture by neutron stars (NSs). If a PBH is
captured by a NS, the star is accreted onto the PBH and gets destroyed in a
very short time. Thus, mere observations of NSs put limits on the abundance of
PBHs. High DM densities and low velocities are required to constrain the
fraction of PBHs in DM. Such conditions may be realized in the cores of
globular clusters if the latter are of a primordial origin. Assuming that cores
of globular clusters possess the DM densities exceeding several hundred
GeV/cm$^3$ would imply that PBHs are excluded as comprising all of the dark
matter in the mass range $3\times 10^{18} \text{g} \lesssim m_\text{BH}\lesssim
10^{24} \text{g}$. At the DM density of $2\times 10^3$ GeV/cm$^3$ that has been
found in simulations in the corresponding models, less than 5% of the DM may
consist of PBH for these PBH masses.
|
Nebular Spectra and Explosion Asymmetry of Type Ia Supernovae | The spectral signatures of asymmetry in Type Ia Supernova (SN Ia) explosions
are investigated, using a sample of late-time nebular spectra. First, a
kinematical model is constructed for SN Ia 2003hv, which can account for the
main features in its optical, Near-Infrared (NIR), and Mid-Infrared (Mid-IR)
late-time spectra. It is found that an asymmetric off-center model can explain
the observed characteristics of SN 2003hv. This model includes a relatively
high density, Fe-rich region which displays a large velocity off-set, and a
relatively low density, extended 56Ni-rich region which is more spherically
distributed. The high density region consists of the inner stable Fe-Ni region
and outer 56Ni-rich region. Such a distribution may be the result of a
delayed-detonation explosion, in which the first deflagration produces the
global asymmetry in the innermost ejecta, while the subsequent detonation can
lead to the bulk spherical symmetry. This configuration, if viewed from the
direction of the off-set, can consistently explain the blueshift in some of the
emission lines and virtually no observed shift in other lines in SN 2003hv. For
this model, we then explore the effects of different viewing angles and the
implications for SNe Ia in general. The model predicts that a variation of the
central wavelength, depending on the viewing angle, should be seen in some
lines (e.g., [Ni II]7378), while the strongest lines (e.g., [Fe III] blend at
4700A) will not show this effect. By examining optical nebular spectra of 12
SNe Ia, we have found that such a variation indeed exists. We suggest that the
global asymmetry in the innermost ejecta, as likely imprint of the deflagration
flame propagation, is a generic feature of SNe Ia (abridged).
|
Multiple Populations in Globular Clusters and the Origin of the
Oosterhoff Period Groups | The presence of multiple populations is now well-established in most globular
clusters in the Milky Way. In light of this progress, here we suggest a new
model explaining the origin of the Sandage period-shift and the difference in
mean period of type ab RR Lyrae variables between the two Oosterhoff groups. In
our models, the instability strip in the metal-poor group II clusters, such as
M15, is populated by second generation stars (G2) with enhanced helium and CNO
abundances, while the RR Lyraes in the relatively metal rich group I clusters
like M3 are mostly produced by first generation stars (G1) without these
enhancements. This population shift within the instability strip with
metallicity can create the observed period-shift between the two groups, since
both helium and CNO abundances play a role in increasing the period of RR Lyrae
variables. The presence of more metal-rich clusters having
Oosterhoff-intermediate characteristics, such as NGC 1851, as well as of most
metal-rich clusters having RR Lyraes with longest periods (group III) can also
be reproduced, as more helium-rich third and later generations of stars (G3)
penetrate into the instability strip with further increase in metallicity.
Therefore, for the most general cases, our models predict that the RR Lyraes
are produced mostly by G1, G2, and G3, respectively, for the Oosterhoff groups
I, II, and III.
|
The ionized and hot gas in M17 SW: SOFIA/GREAT THz observations of [C
II] and 12CO J=13-12 | With new THz maps that cover an area of ~3.3x2.1 pc^2 we probe the spatial
distribution and association of the ionized, neutral and molecular gas
components in the M17 SW nebula. We used the dual band receiver GREAT on board
the SOFIA airborne telescope to obtain a 5'.7x3'.7 map of the 12CO J=13-12
transition and the [C II] 158 um fine-structure line in M17 SW and compare the
spectroscopically resolved maps with corresponding ground-based data for low-
and mid-J CO and [C I] emission. For the first time SOFIA/GREAT allow us to
compare velocity-resolved [C II] emission maps with molecular tracers. We see a
large part of the [C II] emission, both spatially and in velocity, that is
completely non-associated with the other tracers of photon-dominated regions
(PDR). Only particular narrow channel maps of the velocity-resolved [C II]
spectra show a correlation between the different gas components, which is not
seen at all in the integrated intensity maps. These show different morphology
in all lines but give hardly any information on the origin of the emission. The
[C II] 158 um emission extends for more than 2 pc into the M17 SW molecular
cloud and its line profile covers a broader velocity range than the 12CO
J=13-12 and [C I] emissions, which we interpret as several clumps and layers of
ionized carbon gas within the telescope beam. The high-J CO emission emerges
from a dense region between the ionized and neutral carbon emissions,
indicating the presence of high-density clumps that allow the fast formation of
hot CO in the irradiated complex structure of M17 SW. The [C II] observations
in the southern PDR cannot be explained with stratified nor clumpy PDR models.
|
Forward Modeling of Spectroscopic Galaxy Surveys: Application to SDSS | Galaxy spectra are essential to probe the spatial distribution of galaxies in
our Universe. To better interpret current and future spectroscopic galaxy
redshift surveys, it is important to be able to simulate these data sets. We
describe Uspec, a forward modeling tool to generate galaxy spectra taking into
account some intrinsic galaxy properties as well as instrumental responses of a
given telescope. The model for the intrinsic properties of the galaxy
population, i.e., the luminosity functions, and size and spectral coefficients
distribu- tions, was developed in an earlier work for broad-band imaging
surveys [1], and we now aim to test the model further using spectroscopic data.
We apply Uspec to the SDSS/CMASS sample of Luminous Red Galaxies (LRGs). We
construct selection cuts that match those used to build this LRG sample, which
we then apply to data and simulations in the same way. The resulting real and
simulated average spectra show a good statistical agreement overall, with
residual differences likely coming from a bluer galaxy population of the
simulated sam- ple. We also do not explore the impact of non-solar element
ratios in our simulations. For a quantitative comparison, we perform Principal
Component Analysis (PCA) of the sets of spectra. By comparing the PCs
constructed from simulations and data, we find good agree- ment for all
components. The distributions of the eigencoefficients also show an appreciable
overlap. We are therefore able to properly simulate the LRG sample taking into
account the SDSS/BOSS instrumental responses. The differences between the two
samples can be ascribed to the intrinsic properties of the simulated galaxy
population, which can be reduced by further improvements of our modelling
method in the future. We discuss how these results can be useful for the
forward modeling of upcoming large spectroscopic surveys.
|
Young stellar population gradients in central cluster galaxies from NUV
and optical spectroscopy | Central cluster galaxies are the largest and most massive galaxies in the
Universe. Although they host very old stellar populations, several studies
found the existence of blue cores in some BCGs indicating ongoing star
formation. We analyse VLT/X-Shooter stacked spectra of 6 nearby massive central
galaxies with high central velocity dispersions ($\sigma$>300 km/s) at
different galactocentric distances. We quantify the young stellar population
out to 4 kpc by fitting near-UV and optical absorption line indices with
predictions of composite stellar populations modelled by an old and a young
stellar component. We also use IMF-sensitive indices since these galaxies have
been found to host a bottom-heavy IMF in their central regions. We derive
negative young stellar populations gradients, with mass fractions of stars
younger than 1 Gyr decreasing with galactocentric distance, from 0.70% within
0.8 kpc to zero beyond 2 kpc. We also measure the mass fraction in young stars
for individual galaxies in the highest S/N central regions. All the galaxies
have young components of less than one percent. Our results clearly suggest
that the star formation in massive central cluster galaxies takes place in
their galaxy cores (<2 kpc), which, with deeper gravitational potential wells,
are capable of retaining more gas. Among the possible sources for the gas
required to form these young stars, our results are consistent with an in-situ
origin via stellar evolution, which is sufficient to produce the observed young
stellar populations.
|
The Galactic Center Lobe Filled with Thermal Plasma | An observational result of a radio continuum and H92$\alpha$ radio
recombination line of the Galactic Center Lobe (GCL), using the Yamaguchi 32 m
radio telescope, is reported. The obtained spatial intensity distribution of
the radio recombination line shows two distinctive ridge-like structures
extending from the galactic plane vertically to the north at the eastern and
western sides of the galactic center, which are connected to each other at a
latitude of $1.2^{\circ}$ to form a loop-like structure as a whole. This
suggests that most of the radio continuum emission of the GCL is free-free
emission, and that the GCL is filled with thermal plasma. The east ridge of the
GCL observed with the radio recombination line separates 30 pc from the radio
arc, which has been considered as a part of the GCL, but coincides with a ridge
of the radio continuum at a galactic longitude of $0^{\circ}$. The radial
velocity of the radio recombination line is found to be between $-4$ and $+10$
km s$^{-1}$ across the GCL. This velocity is much smaller than the one expected
from the galactic rotation, and hence indicates that the GCL exists apart from
the galactic center. These characteristics of the GCL suggest that the
long-standing hypothesis that the GCL was created by an explosive activity in
the galactic center is unlikely, but favor that the GCL is a giant HII region.
|
Stellar populations in star clusters | Stellar populations contain the most important information about star clus-
ter formation and evolution. Until several decades ago, star clusters were
believed to be ideal laboratories for studies of simple stellar populations
(SSPs). However, discoveries of multiple stellar populations in Galactic
globular clusters have expanded our view on stellar populations in star
clusters. They have simultaneously generated a number of controversies,
particularly as to whether young star clusters may have the same origin as old
globular clusters. In addition, extensive studies have revealed that the SSP
scenario does not seem to hold for some intermediate-age and young star
clusters either, thus making the origin of multiple stellar populations in star
clusters even more complicated. Stellar population anomalies in numerous star
clusters are well-documented, implying that the notion of star clusters as true
SSPs faces serious challenges. In this review, we focus on stellar populations
in massive clusters with different ages. We present the history and progress of
research in this active field, as well as some of the most recent improvements,
including observational results and scenar- ios that have been proposed to
explain the observations. Although our current ability to determine the origin
of multiple stellar populations in star clusters is unsatisfactory, we propose
a number of promising projects that may contribute to a significantly improved
understanding of this subject.
|
IC 630: Piercing the Veil of the Nuclear Gas | IC 630 is a nearby early-type galaxy with a mass of $6 \times 10^{10}
M_{\odot}$ with an intense burst of recent (6 Myr) star formation. It shows
strong nebular emission lines, with radio and X-ray emission, which classifies
it as an AGN. With VLT-SINFONI and Gemini North-NIFS adaptive optics
observations (plus supplementary ANU 2.3m WiFeS optical IFU observations), the
excitation diagnostics of the nebular emission species show no sign of standard
AGN engine excitation; the stellar velocity dispersion also indicate that a
super-massive black hole (if one is present) is small ($M_{\bullet} = 2.25
\times 10^{5}~M_{\odot}$). The luminosity at all wavelengths is consistent with
star formation at a rate of about $1-2 M_{\odot}$/yr. We measure gas outflows
driven by star formation at a rate of $0.18 M_{\odot}$/yr in a face-on
truncated cone geometry. We also observe a nuclear cluster or disk and other
clusters. Photo-ionization from young, hot stars is the main excitation
mechanism for [Fe II] and hydrogen, whereas shocks are responsible for the
H$_2$ excitation. Our observations are broadly comparable with simulations
where a Toomre-unstable, self-gravitating gas disk triggers a burst of star
formation, peaking after about 30 Myr and possibly cycling with a period of
about 200 Myr.
|
MAGIC observations of the giant radio galaxy M87 in a low-emission state
between 2005 and 2007 | We present the results of a long M87 monitoring campaign in very high energy
$\gamma$-rays with the MAGIC-I Cherenkov telescope. We aim to model the
persistent non-thermal jet emission by monitoring and characterizing the very
high energy $\gamma$-ray emission of M87 during a low state. A total of 150\,h
of data were taken between 2005 and 2007 with the single MAGIC-I telescope, out
of which 128.6\,h survived the data quality selection. We also collected data
in the X-ray and \textit{Fermi}--LAT bands from the literature (partially
contemporaneous). No flaring activity was found during the campaign. The source
was found to be in a persistent low-emission state, which was at a confidence
level of $7\sigma$. We present the spectrum between 100\,GeV and 2\,TeV, which
is consistent with a simple power law with a photon index $\Gamma=2.21\pm0.21$
and a flux normalization at 300\,GeV of $(7.7\pm1.3) \times 10^{-8}$ TeV$^{-1}$
s$^{-1}$ m$^{-2}$. The extrapolation of the MAGIC spectrum into the GeV energy
range matches the previously published \textit{Fermi}--LAT spectrum well,
covering a combined energy range of four orders of magnitude with the same
spectral index. We model the broad band energy spectrum with a spine layer
model, which can satisfactorily describe our data.
|
Spatial Distribution and Kinematics of OB Stars | The sample of 37 485 suspected OB stars selected by Gontcharov (2008) from
the Tycho-2 catalogue has been cleaned of the stars that are not of spectral
types OV--A0V. For this purpose, the apparent magnitude $V_T$ from Tycho-2, the
absolute magnitude $M_{V_T}$ calibrated as a function of the dereddened color
index $(B_T-V_T)_0$, the interstellar extinction $A_{V_T}$ calculated from the
3D analytical model by Gontcharov (2009) as a function of the Galactic
coordinates, and the photometric distance $r_{ph}$ calculated as a function of
$V_T$, $M_{V_T}$, and $A_{V_T}$ have been reconciled in an iterative process.
The 20 514 stars that passed the iterations have $(B_T-V_T)_0<0$ and
$M_{V_T}>-5$ and are considered as a sample of OV--A0V stars complete within
350 pc of the Sun. Based on the theoretical relation between the dereddened
color and age of the stars, the derived sample has been divided into three
subsamples: $(B_T-V_T)_0<-0.2^m$, $-0.2^m<(B_T-V_T)_0<-0.1^m$, and
$-0.1^m<(B_T-V_T)_0<0^m$, younger than 100, $100-200$, and $200-400$ Myr,
respectively. The spatial distribution of all 20 514 stars and the kinematics
analyzed for more than 1500 stars with radial velocities from the PCRV and RAVE
catalogues are different for the subsamples, showing smooth rotations, shears,
and deformations of the layer of gas producing stars with the formation of the
Gould Belt, the Great Tunnel, the Local Bubble, and other structures within the
last 200 Myr. The detected temporal variations of the velocity dispersions,
solar motion components, Ogorodnikov--Milne model parameters, and Oort
constants are significant, agree with the results of other authors, and show
that it is meaningless to calculate the kinematic parameters for samples of
stars with uncertain ages or with a wide range of ages.
|
The Atacama Cosmology Telescope: The Two-Season ACTPol
Sunyaev-Zel'dovich Effect Selected Cluster Catalog | We present a catalog of 182 galaxy clusters detected through the
Sunyaev-Zel'dovich effect by the Atacama Cosmology Telescope in a contiguous
987.5 deg$^{2}$ field. The clusters were detected as SZ decrements by applying
a matched filter to 148 GHz maps that combine the original ACT equatorial
survey with data from the first two observing seasons using the ACTPol
receiver. Optical/IR confirmation and redshift measurements come from a
combination of large public surveys and our own follow-up observations. Where
necessary, we measured photometric redshifts for clusters using a pipeline that
achieves accuracy $\Delta z/(1 + z)=0.015$ when tested on SDSS data. Under the
assumption that clusters can be described by the so-called Universal Pressure
Profile and its associated mass-scaling law, the full signal-to-noise > 4
sample spans the mass range $1.6 < M^{\rm UPP}_{\rm 500c}/10^{14}{\rm
M}_{\odot}<9.1$, with median $M^{\rm UPP}_{\rm 500c}=3.1 \times 10^{14}$
M$_{\odot}$. The sample covers the redshift range $0.1 < z < 1.4$ (median $z =
0.49$) and 28 clusters are new discoveries (median $z = 0.80$). We compare our
catalog with other overlapping cluster samples selected using the SZ,
optical,and X-ray wavelengths. We find the ratio of the UPP-based SZ mass to
richness-based weak-lensing mass is $\langle M^{\rm UPP}_{\rm 500c} \rangle /
\langle M^{\rm \lambda WL}_{\rm 500c} \rangle = 0.68 \pm 0.11$. After applying
this calibration, the mass distribution for clusters with $M_{\rm 500c} > 4
\times 10^{14}$ M$_{\odot}$ is consistent with the number of such clusters
found in the South Pole Telescope SZ survey.
|
First detection of VHE gamma-rays from SN 1006 by H.E.S.S | Recent theoretical predictions of the lowest very high energy (VHE)
luminosity of SN 1006 are only a factor 5 below the previously published
H.E.S.S. upper limit, thus motivating further in-depth observations of this
source. Deep observations at VHE energies (above 100 GeV) were carried out with
the High Energy Stereoscopic System (H.E.S.S.) of Cherenkov Telescopes from
2003 to 2008. More than 100 hours of data have been collected and subjected to
an improved analysis procedure. Observations resulted in the detection of VHE
gamma-rays from SN 1006. The measured gamma-ray spectrum is compatible with a
power-law, the flux is of the order of 1% of that detected from the Crab
Nebula, and is thus consistent with the previously established H.E.S.S. upper
limit. The source exhibits a bipolar morphology, which is strongly correlated
with non-thermal X-rays. Because the thickness of the VHE-shell is compatible
with emission from a thin rim, particle acceleration in shock waves is likely
to be the origin of the gamma-ray signal. The measured flux level can be
accounted for by inverse Compton emission, but a mixed scenario that includes
leptonic and hadronic components and takes into account the ambient matter
density inferred from observations also leads to a satisfactory description of
the multi-wavelength spectrum.
|
The Stochastic Properties of $\ell^1$-Regularized Spherical Gaussian
Fields | Convex regularization techniques are now widespread tools for solving inverse
problems in a variety of different frameworks. In some cases, the functions to
be reconstructed are naturally viewed as realizations from random processes; an
important question is thus whether such regularization techniques preserve the
properties of the underlying probability measures. We focus here on a case
which has produced a very lively debate in the cosmological literature, namely
Gaussian and isotropic spherical random fields, and we prove that Gaussianity
and isotropy are not conserved in general under convex regularization over a
Fourier dictionary, such as the orthonormal system of spherical harmonics.
|
Fayet-Iliopoulos terms in supergravity and D-term inflation | We analyse the consequences of a new gauge invariant Fayet-Iliopoulos (FI)
term proposed recently to a class of inflation models driven by supersymmetry
breaking with the inflaton being the superpartner of the goldstino. We first
show that charged matter fields can be consistently added with the new term, as
well as the standard FI term in supergravity in a K\"ahler frame where the
$U(1)$ is not an R-symmetry. We then show that the slow-roll conditions can be
easily satisfied with inflation driven by a D-term depending on the two FI
parameters. Inflation starts at initial conditions around the maximum of the
potential where the $U(1)$ symmetry is restored and stops when the inflaton
rolls down to the minimum describing the present phase of our Universe. The
resulting tensor-to-scalar ratio of primordial perturbations can be even at
observable values in the presence of higher order terms in the K\"ahler
potential.
|
Magnetic Fields in Molecular Clouds -- Observation and Interpretation | The Zeeman effect and dust grain alignment are two major methods for probing
magnetic fields (B-fields) in molecular clouds, largely motivated by the study
of star formation, as the B-field may regulate gravitational contraction and
channel turbulence velocity. This review summarizes our observations of
B-fields over the past decade, along with our interpretation. Galactic B-fields
anchor molecular clouds down to cloud cores with scales around 0.1 pc and
densities of 10^4-5 H_2/cc. Within the cores, turbulence can be slightly
super-Alfvenic, while the bulk volumes of pa-rental clouds are sub-Alfvenic.
The consequences of these largely ordered cloud B-fields on fragmentation and
star formation are observed. The above paradigm is very different from the
generally accepted theory during the first decade of the century, when cloud
turbulence was assumed to be highly super-Alfvenic. Thus, turbulence anisotropy
and turbulence-induced ambipolar diffusion are also revisited.
|
Tests of gravity theories with Galactic Center observations | An active stage of relativistic astrophysics started in 1963 since in this
year, quasars were discovered, Kerr solution has been found and the first Texas
Symposium on Relativistic Astrophysics was organized in Dallas. Five years
later, in 1967--1968 pulsars were discovered and their model as rotating
neutron stars has been proposed, meanwhile J. A. Wheeler claimed that Kerr and
Schwarzschild vacuum solutions of Einstein equations provide an efficient
approach for astronomical objects with different masses. Wheeler suggested to
call these objects black holes. Neutron stars were observed in different
spectral band of electromagnetic radiation. In addition, a neutrino signal has
been found for SN1987A. Therefore, multi-messenger astronomy demonstrated its
efficiency for decades even before observations of the first gravitational
radiation sources. However, usually, one has only manifestations of black holes
in a weak gravitational field limit and sometimes a model with a black hole
could be substituted with an alternative approach which very often looks much
less natural, however, it is necessary to find observational evidences to
reject such an alternative model. After two observational runs the LIGO-- Virgo
collaboration provided a confirmation for an presence of mergers for ten binary
black holes and one binary neutron star system where gravitational wave signals
were found. In addition, in last years a remarkable progress has been reached
in a development of observational facilities to investigate a gravitational
potential, for instance, a number of telescopes operating in the Event Horizon
Telescope network is increasing and accuracy of a shadow reconstruction near
the Galactic Center is improving, meanwhile largest VLT, Keck telescopes with
adaptive optics and especially, GRAVITY facilities observe bright IR stars at
the Galactic Center with a perfecting accuracy.
|
Cosmic ray transport in partially turbulent space plasmas with
compressible magnetic turbulence | Recently a new transport theory of cosmic rays in magnetized space plasmas
extending the quasilinear approximation to the particle orbit has been
developed for the case of an axisymmetric incompressible magnetic turbulence.
Here we generalize the approach to the important physical case of a
compressible plasma. As previously obtained in the case of an incompressible
plasma we allow arbitrary gyrophase deviations from the unperturbed spiral
orbits in the uniform magnetic field. For the case of quasi-stationary and
spatially homogeneous magnetic turbulence we derive in the small Larmor radius
approximation gyro-phase averaged cosmic ray Fokker-Planck coefficients. Upper
limits for the perpendicular and pitch-angle Fokker-Planck coefficients and for
the perpendicular and parallel spatial diffusion coefficients are presented.
|
Photometric and Spectroscopic Properties of Type II-P Supernovae | We study a sample of 23 Type II Plateau supernovae (SNe II-P), all observed
with the same set of instruments. Analysis of their photometric evolution
confirms that their typical plateau duration is 100 days with little scatter,
showing a tendency to get shorter for more energetic SNe. The rise time from
explosion to plateau does not seem to correlate with luminosity. We analyze
their spectra, measuring typical ejecta velocities, and confirm that they
follow a well behaved power-law decline. We find indications of high-velocity
material in the spectra of six of our SNe. We test different dust extinction
correction methods by asking the following -- does the uniformity of the sample
increase after the application of a given method? A reasonably behaved
underlying distribution should become tighter after correction. No method we
tested made a significant improvement.
|
Disk-mediated accretion burst in a high-mass young stellar object | Solar-mass stars form via circumstellar disk accretion (disk-mediated
accretion). Recent findings indicate that this process is likely episodic in
the form of accretion bursts, possibly caused by disk fragmentation. Although
it cannot be ruled out that high-mass young stellar objects (HMYSOs; $M>$8
M$_\odot$, $L_{bol}>$5$\times$10$^3$ L$_\odot$) arise from the coalescence of
their low-mass brethren, latest results suggest that they more likely form via
disks. Accordingly, disk-mediated accretion bursts should occur. Here we report
on the discovery of the first disk-mediated accretion burst from a $\sim$20
M$_\odot$ HMYSO. Our near-infrared images show the brightening of the central
source and its outflow cavities. Near-infrared spectroscopy reveals emission
lines typical of accretion bursts in low-mass protostars, but orders of
magnitude more luminous. Moreover, the energy released and the inferred
mass-accretion rate are also orders of magnitude larger. Our results identify
disk accretion as the common mechanism of star formation across the entire
stellar mass spectrum.
|
Extragalactic background Light: a measurement at 400 nm using dark cloud
shadow II. Spectroscopic separation of dark cloud's light, and results | In a project aimed at measuring the optical Extragalactic Background Light
(EBL) we are using the shadow of a dark cloud.We have performed, with the ESO
VLT/FORS, spectrophotometry of the surface brightness towards the
high-galactic-latitude dark cloud Lynds 1642. A spectrum representing the
difference between the opaque core of the cloud and several unobscured
positions around the cloud was presented in Paper I (Mattila et al. 2017a). The
topic of the present paper is the separation of the scattered starlight from
the dark cloud itself which is the only remaining foreground component in this
difference. While the scattered starlight spectrum has the characteristic
Fraunhofer lines and the discontinuity at 400 nm, typical of integrated light
of galaxies, the EBL spectrum is a smooth one without these features. As
template for the scattered starlight we make use of the spectra at two
semi-transparent positions. The resulting EBL intensity at 400 nm is $I_{\rm
EBL} = 2.9\pm1.1$ $10^{-9}$ erg cm$^{-2}$s$^{-1}$sr$^{-1}$\AA$^{-1}$, or
$11.6\pm4.4$ nW m$^{-2}$sr$^{-1}$, which represents a 2.6$\sigma$ detection;
the scaling uncertainty is +20%/-16%. At 520 nm we have set a 2$\sigma$ upper
limit of $I_{\rm EBL} \le$4.5 $10^{-9}$ erg
cm$^{-2}$s$^{-1}$sr$^{-1}$\AA$^{-1}$ or $\le$23.4 nW m$^{-2}$sr$^{-1}$
+20%/-16%. Our EBL value at 400 nm is $\ge 2$ times as high as the integrated
light of galaxies. No known diffuse light sources, such as light from Milky Way
halo, intra-cluster or intra-group stars appear capable of explaining the
observed EBL excess over the integrated light of galaxies.
|
Classification of cosmology with arbitrary matter in the
Ho\v{r}ava-Lifshitz theory | In this short note, we are going to classify the cosmological evolutions in
the recently proposed, nonrelativistic gravitational theory, the
Ho\v{r}ava-Lifshitz (HL) theory. We consider the original HL theory (theory I),
and the modified version obtained by an analytic continuation of parameters
(theory II). We discuss the possiblilities of cosmological evolutions with
arbitrary matter.
|
Rich structure of non-thermal relativistic CMB spectral distortions from
high energy particle cascades at redshifts $z\lesssim 2\times 10^5$ | It is generally assumed that for energy injection before recombination, all
of the injected energy is dissipated as heat in the baryon-photon plasma,
giving rise to the $y$-type, $i$-type, and $\mu$-type distortions in the CMB
spectrum. We show that this assumption is incorrect when the energy is injected
in the form of energetic (i.e. energy much greater than the background CMB
temperature) particles. We evolve the electromagnetic cascades, from the
injection of high energy particles, in the expanding Universe and follow the
non-thermal component of CMB spectral distortions resulting from the
interaction of the electromagnetic shower with the background photons,
electrons, and ions. The electromagnetic shower loses a substantial fraction of
its energy to the CMB spectral distortions before the energy of the particles
in the shower has degraded to low enough energies that they can thermalize with
the background plasma. This spectral distortion is the result of the
interaction of non-thermal energetic electrons in the shower with the CMB and
thus has a shape that is substantially different from the $y$-type or $i$-type
distortions. The shape of the final \emph{non-thermal relativistic}
($ntr$-type) CMB spectral distortion depends upon the initial energy spectrum
of the injected electrons, positrons, and photons and thus has information
about the energy injection mechanism e.g. the decay or annihilation channel of
the decaying or annihilating dark matter particles. The shape of the spectral
distortion is also sensitive to the redshift of energy injection. Our
calculations open up a new window into the energy injection at $z\lesssim
2\times 10^5$ which is not degenerate with, and can be distinguished from the
low redshift thermal $y$-type distortions.
|
Ghost condensation and CPT violation in neutrino sector | We consider CPT violation in neutrino sector, which is induced by ghost
condensation. A model with extra dimension is suggested where ghost
condensation occurs at a distant location separated from the SM brane. Right
handed neutrinos in the bulk, which are originally introduced to explain small
Yukawa couplings, play the role of messenger fields communicating ghost
condensation and the standard model sector and lead to a sizable CPT violation
in neutrino sector at the leading order. The model provides a resolution to the
recent MINOS anomaly without spoiling any experimental constraints and may be
able to be tested by observing an interesting phenomenon, twinkling cosmic
microwave background radiation, with timescale about O(10-100) minutes at
future CMB observations e.g. Planck.
|
Zipping and Unzipping of Cosmic String Loops in Collision | In this paper the collision of two cosmic string loops is studied. After
collision junctions are formed and the loops are entangled. We show that after
their formation the junctions start to unzip and the loops disentangle. This
analysis provides a theoretical understanding of the unzipping effect observed
in numerical simulations of a network of cosmic strings with more than one type
of cosmic strings. The unzipping phenomena have important effects in the
evolution of cosmic string networks when junctions are formed upon collision,
such as in a network of cosmic superstrings.
|
Type 2 AGN host galaxies in the Chandra-COSMOS Legacy Survey: No
Evidence of AGN-driven Quenching | We investigate the star formation properties of a large sample of ~2300
X-ray-selected Type 2 Active Galactic Nuclei (AGNs) host galaxies out to z~3 in
the Chandra COSMOS Legacy Survey in order to understand the connection between
the star formation and nuclear activity. Making use of the existing
multi-wavelength photometric data available in the COSMOS field, we perform a
multi-component modeling from far-infrared to near-ultraviolet using a nuclear
dust torus model, a stellar population model and a starburst model of the
spectral energy distributions (SEDs). Through detailed analysis of SEDs, we
derive the stellar masses and the star formation rates (SFRs) of Type 2 AGN
host galaxies. The stellar mass of our sample is in the range 9 < log
M_{stellar}/M_{\odot} < 12 with uncertainties of ~0.19 dex. We find that Type 2
AGN host galaxies have, on average, similar SFRs compared to the normal
star-forming galaxies with similar M_{stellar} and redshift ranges, suggesting
no significant evidence for enhancement or quenching of star formation. This
could be interpreted in a scenario, where the relative massive galaxies have
already experienced substantial growth at higher redshift (z>3), and grow
slowly through secular fueling processes hosting moderate-luminosity AGNs.
|
Simulating the infrared sky with a Spritz | Current hydrodynamical and semi-empirical simulations of galaxy formation and
evolution have difficulties in reproducing the number densities of IR-detected
galaxies. Therefore, a versatile, phenomenological new simulation tool is
necessary to reproduce current and predict future observations at IR
wavelengths. In this work we generate simulated catalogues starting from the
Herschel infrared luminosity functions of different galaxy populations, in
order to consider in a consistent way different populations of galaxies and
active galactic nuclei. We associated a spectral energy distribution and
physical properties, such as stellar mass, star-formation-rate and AGN
contribution, to each simulated galaxy using a broad set of empirical
relations. We compare the resulting simulated galaxies, extracted up to z$=$10,
with a broad set of observational relations. The Spectro-Photometric
Realisations of Infrared-selected Targets at all-z (SPRITZ) simulation will
allow us to obtain in a fully consistent way simulated observations for a broad
set of current and future facilities with photometric capabilities as well as
low-resolution IR spectroscopy, like the James Webb Space Telescope (JWST) or
the Origin Space Telescope (OST). The derived simulated catalogue contains
galaxies and active galactic nuclei that by construction reproduce the observed
IR galaxy number density, but it is also in agreement with the observed number
counts from UV to far-IR wavelengths, the observed stellar mass function, the
star-formation-rate vs. stellar mass plane and the luminosity function from the
radio to the X-ray. The proposed simulation is therefore ideal to make
predictions for current and future facilities, in particular, but not limited
to, those operating at IR wavelengths. The SPRITZ simulation will be publicly
available.
|
Statistical Microlensing Toward Magnified High-Redshift Star Clusters | We study light variability of gravitationally magnified high-redshift star
clusters induced by a foreground population of microlenses. This arises as the
incoherent superposition of light variations from a large number of source
stars traversing the random magnification pattern on the source plane. The
light curve resembles a scale-invariant, Gaussian process on timescales of
years to decades, while exhibits rapid and frequent micro-caustic crossing
flares of larger amplitudes on timescales of days to months. For a concrete
example, we study a young Lyman-continuum-leaking star cluster recently
discovered in the lensed Sunburst Arc at $z=2.37$. We show that one magnified
image happens to be intervened by a faint foreground galaxy, and hence should
exhibit a variable flux at the $1$--$2\%$ level, which is measurable in space
with $\sim 1$--$3\,$ks exposures on the Hubble Space Telescope and more easily
with the James Webb Space Telescope, or from the ground using a $\sim$4-meter
telescope without adaptive optics. Detailed measurement of this variability
will enable us to determine the absolute macro magnification and hence the
intrinsic mass and length scales of the star cluster, test synthetic models of
stellar population, and probe multiplicity of massive stars. We furthermore
suggest that monitoring the other lensed images of the star cluster, which are
free from significant intervention by foreground stellar microlenses, will
allow us to probe planetary to stellar mass compact objects constituting as
little as just a few percent of the dark matter. Given the typical surface
density of intracluster stars, we expect this phenomenon to be relevant for
many other gravitationally magnified star clusters at Cosmic Noon behind galaxy
cluster lenses.
|
Non-Linear Matter Power Spectrum Covariance Matrix Errors and
Cosmological Parameter Uncertainties | The covariance matrix of the matter power spectrum is a key element of the
statistical analysis of galaxy clustering data. Independent realisations of
observational measurements can be used to sample the covariance, nevertheless
statistical sampling errors will propagate into the cosmological parameter
inference potentially limiting the capabilities of the upcoming generation of
galaxy surveys. The impact of these errors as function of the number of
independent realisations has been previously evaluated for Gaussian distributed
data. However, non-linearities in the late time clustering of matter cause
departures from Gaussian statistics. Here, we address the impact of
non-Gaussian errors on the sample covariance and precision matrix errors using
a large ensemble of numerical N-body simulations. In the range of modes where
finite volume effects are negligible ($0.1\lesssim k\,[h\,{\rm
Mpc^{-1}}]\lesssim 1.2$) we find deviations of the estimated variance of the
sample covariance with respect to Gaussian predictions above $\sim 10\%$ level.
These reduce to about $\sim 5\%$ in the case of the precision matrix. Finally,
we perform a Fisher analysis to estimate the effect of covariance errors on the
cosmological parameter constraints. In particular, assuming Euclid-like survey
characteristics we find that a number of independent realisation larger than
$\gtrsim 5000$ is necessary to reduce the contribution of sample covariance
errors to the cosmological parameter uncertainties at sub-percent level. We
also show that restricting the analysis to large scales $k\lesssim0.2\,h\,{\rm
Mpc^{-1}}$ results in a considerable loss in constraining power, while using
the linear covariance to include smaller scales leads to an underestimation of
the errors on the cosmological parameters.
|
Halo bias in the excursion set approach with correlated steps | In the Excursion Set approach, halo abundances and clustering are closely
related. This relation is exploited in many modern methods which seek to
constrain cosmological parameters on the basis of the observed spatial
distribution of clusters. However, to obtain analytic expressions for these
quantities, most Excursion Set based predictions ignore the fact that, although
different k-modes in the initial Gaussian field are uncorrelated, this is not
true in real space: the values of the density field at a given spatial
position, when smoothed on different real-space scales, are correlated in a
nontrivial way. We show that when the excursion set approach is extended to
include such correlations, then one must be careful to account for the fact
that the associated prediction for halo bias is explicitly a real-space
quantity. Therefore, care must be taken when comparing the predictions of this
approach with measurements in simulations, which are typically made in
Fourier-space. We show how to correct for this effect, and demonstrate that
ignorance of this effect in recent analyses of halo bias has led to incorrect
conclusions and biased constraints.
|
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.