---
abstract: 'The luminous $z=0.286$ quasar [HE0450–2958]{} is interacting with a companion galaxy at 6.5 kpc distance and the whole system radiates in the infrared at the level of an ultraluminous infrared galaxy (ULIRG). A so far undetected host galaxy triggered the hypothesis of a mostly “naked” black hole (BH) ejected from the companion by three-body interaction. We present new HST/NICMOS 1.6$\mu$m imaging data at 01 resolution and VLT/VISIR 11.3$\mu$m images at 035 resolution that are for the first time resolving the system in the near- and mid-infrared. We combine these data with existing optical HST and CO maps. (i) At 1.6$\mu$m we find an extension N-E of the quasar nucleus that is likely a part of the host galaxy, though not its main body. If true, a combination with upper limits on a main body co-centered with the quasar brackets the host galaxy luminosity to within a factor of $\sim$4 and places [HE0450–2958]{} directly onto the $M_\mathrm{BH}-M_\mathrm{bulge}$-relation for nearby galaxies. (ii) A dust-free line of sight to the quasar suggests a low dust obscuration of the host galaxy, but the formal upper limit for star formation lies at 60 M$_\odot$/yr. [HE0450–2958]{} is consistent with lying at the high-luminosity end of Narrow-Line Seyfert 1 Galaxies, and more exotic explanations like a “naked quasar” are unlikely. (iii) All 11.3$\mu$m radiation in the system is emitted by the quasar nucleus. It has warm ULIRG-strength IR emission powered by black hole accretion and is radiating at super-Eddington rate, $L/L_\mathrm{Edd}=6.2^{+3.8}_{-1.8}$, or 12 $M_\odot$/year. (iv) The companion galaxy is covered in optically thick dust and is not a collisional ring galaxy. It emits in the far infrared at ULIRG strength, powered by Arp220-like star formation (strong starburst-like). An M82-like SED is ruled out. (v) With its black hole accretion rate [HE0450–2958]{} produces not enough new stars to maintain its position on the $M_\mathrm{BH}-M_\mathrm{bulge}$-relation, and star formation and black hole accretion are spatially disjoint. This relation can either only be maintained averaging over a longer timescale ($\la$500 Myr) and/or the bulge has to grow by redistribution of preexisting stars. (vi) Systems similar to [HE0450–2958]{} with spatially disjoint ULIRG-strength star formation and quasar activity might be common at high redshifts but at $z<0.43$ we only find $<$4% (3/77) candidates for a similar configuration.'
author:
- 'Knud Jahnke, David Elbaz, Eric Pantin, Asmus Böhm$^,$, Lutz Wisotzki, Geraldine Letawe, Virginie Chantry, Pierre-Olivier Lagage'
title: |
    The QSO HE0450–2958: Scantily dressed or heavily robed?\
    A normal quasar as part of an unusual ULIRG.
---

Introduction
============

In the current framework of galaxy evolution, galaxies and black holes are intimately coupled in their formation and evolution. The masses of galactic bulges and their central black holes (BHs) in the local Universe follow a tight relation [e.g. @haer04] with only 0.3 dex scatter. Currently it is not clear how this relation comes about and if and how it evolved over the last 13 Gyrs, but basically all semi-analytic models now include feedback from active galactic nuclei (AGN) as a key ingredient to acquire consensus with observations [e.g. @hopk06c; @some08]. In these models it is assumed that black hole growth by accretion and energetic re-emission from the ignited AGN back into the galaxy can form a self regulating feedback chain. This feedback loop can potentially regulate or possibly also truncate star formation and in this process create and maintain the red/blue color–magnitude bimodality of galaxies. In this light, any galaxy with an abnormal deviation from the $M_\mathrm{BH}$–$M_\mathrm{bulge}$-relation will be an important laboratory for understanding the coupling mechanisms of black hole and bulge growth. It will set observational limits for these models, and constrain the time-lines and required physics involved.

Since the early work by @bahc94 [@bahc95b] on QSO host galaxies with the [*Hubble Space Telescope (HST)*]{} and the subsequently resolved dispute about putatively “naked” QSOs [@mcle95a], no cases for QSOs without surrounding host galaxies were found – when detection limits were correctly interpreted. Only recently the QSO [HE0450–2958]{} renewed the discussion, when @maga05 made a case for a 6$\times$ too faint upper limit of the host galaxy of [HE0450–2958]{} with respect to the $M_\mathrm{BH}$–$M_\mathrm{bulge}$-relation. In light of a number of competing explanations for this, the nature of the [HE0450–2958]{} system needs to be settled.

The QSO [HE0450–2958]{} (a.k.a. IRAS 04505–2958) at a redshift of $z=0.286$ was discovered by @low88 as a warm IRAS source. [HE0450–2958]{} is a radio-quiet quasar, with a distorted companion galaxy at 15 (=6.5 kpc) distance at the same redshift, likely in direct interaction with the QSO [@cana01]. The combined system shows an infrared luminosity of an ultraluminous infrared galaxy (ULIRG, $L_\mathrm{IR}>10^{12}$ L$_\odot$).

[HE0450–2958]{} was observed with the [Hubble Space Telescope (HST)]{} and its WFPC2 camera [@boyc96] in F702W (=$R$ band) and ACS camera [@maga05] in F606W (=$V$ band), both observations did not allow to detect a host galaxy centered on the quasar position within their limits (Figure \[fig:allwave\], left column). @maga05 estimated an expected host galaxy brightness if [HE0450–2958]{} was a normal QSO system that obeyed the $M_\mathrm{BH}$–$M_\mathrm{bulge}$-relation in the local Universe and given a BH mass estimate or luminosity of the QSO. They concluded that the ACS F606W detection limits were six times fainter than the expected value for the host galaxy, which qualified [HE0450–2958]{} to be very unusual.

![image](fig1_all.eps){width="\textwidth"}

@maga05 sparked a flurry of subsequent papers to explain the undetected host galaxy to black hole relation. Over time three different alternative explanations have been put forward and were substantiated:

1.  [HE0450–2958]{} is a normal QSO nucleus, but with a massive black hole residing in an under-massive host galaxy. The system is lying substantially off the local $M_\mathrm{BH}$–$M_\mathrm{bulge}$-relation; the host galaxy possibly hides just below the F606W detection limit [@maga05].

2.  The host galaxy is actually absent, [HE0450–2958]{} is a truly “naked” QSO, by means of a black hole ejection event in a gravitational three body interaction or gravitational recoil following the merger of [HE0450–2958]{} with the companion galaxy [@hoff06; @haeh06; @bonn07].

3.  The original black hole mass estimate was too high [@merr06; @kim07; @leta07] and is in fact $\sim$10 times lower. With comparably narrow ($\sim$1500 km/s FWHM) broad QSO emission lines the QSO could be the high-luminosity analog of the class of narrow-line Seyfert 1 galaxies (NLSy1). The host galaxy could be normal for the black hole mass and be absolutely consistent with the ACS upper limits.

In this article we present new data initially motivated by the still undetected host galaxy and by the possibility that the host galaxy might be obscured by substantial amounts of dust. We want to investigate the overall cool and warm dust properties of the system, using new near infrared (NIR) and mid infrared (MIR) images. The F606W ACS band is strongly susceptible to dust attenuation, and dust could have prevented the detection of the host galaxy in the optical. With new NIR data we look at a substantially more transparent wavelength.

At the same time the new infrared data is meant to localize the source(s) of the ULIRG emission. Three components are candidates for this: The AGN nucleus, the host galaxy, and the companion galaxy. Our NIR data allow to trace star formation and the MIR image traces the hot dust in the system. We present the new data and interpret it in the view of the so far collected knowledge from X-ray to radio-wavelengths that was built up since the article of @maga05.

Throughout we will use Vega zero-points and a cosmology of $h=H_0/(100\mathrm{km s^{-1} Mpc^{-1}})=0.7$, $\Omega_M=0.3$, and $\Omega_\Lambda=0.7$, corresponding to a distance modulus of 40.84 for $z=0.286$ and linear scales of 4.312 kpc/.

The IR angle
============

Up to now the only existing infrared observations on [HE0450–2958]{} were from the 2MASS survey in the near infrared $J$, $H$, and $K$ bands at $\sim$4resolution, and in the MIR from the IRAS mission [@grij87; @low88] at 12, 25, 60, and 100 $\mu$m with about 4 resolution. Both surveys do not resolve the different individual components of the system (QSO, companion galaxy, foreground star). De Grijp et al. ([-@grij87]) noted that the [HE0450–2958]{} system is showing the MIR/FIR luminosities of a ULIRG system, but it was not clear which components of the system are responsible for this emission due to the coarse IRAS resolution. We want to localize the dust emission in two ways: (a) A direct observation of the hot dust component at 8.9$\mu$m (rest-frame) with the VISIR imager at the ESO VLT. (b) A localization of dust in general by combining new HST near infrared and the existing ACS optical data. For this purpose we obtained HST NIC2 imaging in the rest-frame $J$-band at $\sim$1.3$\mu$m.

VISIR 11.3$\mu$m imaging data
-----------------------------

In the near and mid infrared the [HE0450–2958]{} system clearly has a spectral energy distribution (SED) that is composed of more than a single component: In Figure \[fig:iras\_sed\] we model the IRAS and VISIR flux densities with a composite SED of a quasar plus a star forming component. For the quasar we test the median and 68 percentile reddest quasar SED from @elvi94, and for the star forming component an Arp220-like starbursting SED, but we also tried a medium star formation M82 SED, both from @elba02. The median quasar SED plus Arp220 can reproduce the data at all wavlengths, except at observed 25$\mu$m, where it leaves a small mismatch. The 68 percentile reddest SED on the other hand creates a perfect match also there. For both cases the flux predicted for the companion galaxy at 11.3$\mu$m lies below the detection limit as observed. Milder, M82-like star formation can be ruled out on the same basis, as it predicts a detection of the companion also at 11.3$\mu$m – both with the information from the mid infrared, as well as when extrapolating the observed $H$-band flux.

@papa08 match a simple model of two black-body emission curves to the four IRAS points, yielding a cool dust component heated by star formation and a warm dust component which can be attributed to intense AGN emission (see Section \[sec:discussion\_ulirg\]). While it is not possible to spatially resolve the system at FIR wavelengths with current telescopes, we aim for the highest wavelength where this is currently possible, in order to localize the warm emission component and test whether this comes solely from the (optically visible) QSO or from extra sources.

![\[fig:iras\_sed\] The SED of [HE0450–2958]{} in the mid-infrared: Shown are the IRAS flux density measurements from @grij87 [*(open circles)*]{}, our VISIR data point [*(filled circle)*]{} and upper limit on the companion galaxy [*(arrow)*]{} and overlaid composite AGN plus starburst SEDs [*(lines)*]{}. For the quasar nucleus we use the median [*(green dashed line)*]{} and 68 percentile reddest SEDs [*(black dashed line)*]{} from @elvi94, the starburst [*(red solid line)*]{} is a model for Arp220 by @elba02. The median quasar plus Arp220 SED [*(green solid line)*]{} can explain the data except for a slightly too low value at the observed 25$\mu$m point, but with the 68 percentile SED [*(black solid line)*]{} the match is perfect. The predicted flux of the companion galaxy where the star formation of the system is located [*(bar)*]{} lies below our detection limit, consistent with the data. Milder star formation templates as e.g. M82 can be ruled out, since they predict too high fluxes for the companion – also from the observed $H$-band data – which should be visible in the VISIR image. ](fig2.eps){width="\columnwidth"}

The observations were performed using VISIR, the ESO/VLT mid-infrared imager and spectrograph mounted on unit 3 of the VLT (Melipal). VISIR gives a pixel size of 0075 and a total field-of-view of 192. The diffraction limited resolution is 035 FWHM. Standard “chopping and nodding” mid-infrared observational technique was used to supress the background dominating at these wavelength. All the observations were interlaced with standard star observations of HD 29085 (4.45 Jy) and HD 41047 (7.21 Jy). The estimated sensitivity was 4 mJy/10$\sigma$/1h.

Imaging data were obtained on the 12th of December 2005 in service observing mode, through the PAH2 filter centered on 11.3 $\mu$m having a half-band width of 0.6 $\mu$m. Weather conditions were very good, optical seeing was below 1, and the object was observed always at an airmass of 1.15, which resulted in a diffraction limited image of 035 resolution. Chopping/nodding parameters were 8/8and 0.25 Hz/0.033 Hz. The total time spent on-source was 1623 s. The data were reduced using a dedicated pipeline written in IDL, which does the chopping/nodding correction and removes the spurious stripes due to detector instabilities [@pant07]. The reduced data were finally flux-calibrated using the two reference stars as photometric calibrators. The error on the photometry due to variations of the atmospheric transmission are estimated to be less than 2% (3$\sigma$).

NICMOS $H$-band imaging data
----------------------------

The ACS $V$-band is too blue to penetrate any substantial amount of dust. With the scenario of a dust enshrouded host galaxy in mind, we acquired new HST NICMOS data (NIC2 with 0075 plate scale) in the F160W $H$-band (program \#10797, cycle 15) to reduce the dust attenuation by a factor of 3.5 in magnitude space.

A total of 5204s integration on target was forcedly split into two observation attempts due to telescope problems, and carried out in July 2006 and 2007. These yielded two sets of data with 2602 s integration each, but slightly different orientations. In order to minimize chromatic effects, we also observed a point spread function (PSF) calibrator star (EIS J033259.33–274638.5) with the SED-characteristics over the F160W filter bandpass similar to a mean QSO template. We do not know the actual SED of [HE0450–2958]{} itself, as no NIR imaging or spectroscopic data of the system with high enough spatial resolution exist to date. As the stellar type yielding the likely most similar PSF we found K4III, by comparing the PSFs predicted by the TinyTim package [@kris03]. The only cataloged stars faint enough to not immediately saturate were observed by the ESO Imaging Survey [EIS, @groe02] located in the E-CDFS, and had to be observed at 6 months distance in time to [HE0450–2958]{}. Since we also want to minimize the PSF variation due to differences in observing strategy, we applied the same dither patterns for both [HE0450–2958]{} and the PSF star. Due to the absolute pointing accuracy of HST the centroid location of the star relative to the chip is shifted about 15 pixels (11) from the QSO centroid towards the companion galaxy.

Data reduction and combination of the individual frames were carried out using a mix of STScI pipeline data products, pyraf, and our own procedures in MIDAS and Fortran. The resulting image is shown together with the analysis in Figure \[fig:dataimages\]a. Two parts of the team analyzed the combined images in complementary ways, by decomposition of the components using two-dimensional modeling and by image deconvolution.

### Uncertainty in the PSF {#sec:psfuncertainty}

In order to detect a putative faint host galaxy underneath the bright QSO nucleus we require a precise knowledge of the PSF. The PSF will vary spatially, with the energy distribution in the filter as well as temporally, with a changing effective focus of the telescope due to changing thermal history.

We opt for a double approach: First, we observe the separate PSF star with the properties described in the last section (and see Fig. \[fig:dataimages\]b). Secondly, we also have the foreground star available that is located at 18 distance from the QSO to the north-west. It is classified as a G star [@low89]. Its on-chip distance to the QSO will leave only room for small spatial variations, but its SED in the $H$-band likely will not perfectly match the SED of the QSO.

It is difficult to assess the PSF uncertainty at the position of the QSO. In principle we have a combined effect of color, spatial, and temporal variation, but only one bit of information: the difference between the foreground star and the PSF star. We thus model the expected difference in the shape of these two stars with TinyTim and then compare their actual observed shapes. This shows that the foreground star should be slightly narrower than the observed PSF star, which is consistent with PSF star’s later, redder spectral type and an increase of PSF width with wavelength. We observe this effect also in the data, however somewhat stronger. A temporal variation can thus not be separated and ruled out.

In any case we conclude that the PSF star is wider and thus will yield more conservative (=fainter) estimates for a QSO host galaxy, while in case of a non-detection the foreground star will yield brighter upper limits.

For two-dimensional modeling of the system we use [galfit]{} [@peng02]. In order to quantify the PSF uncertainty for this process, we first let [ galfit]{} fit a single point source, represented by the PSF star, to the foreground star. In this process we use an error map created from the data itself and we add the sky as a free parameter. We minimize the influence of the nearby QSO on the foreground star by first fitting the former with a single point source as well, removing its modeled contribution, and mask out the remaining residuals starting at 09 from the star. The PSF created in this way is shown in Figure \[fig:dataimages\]c. This image is fed into the modeling process of the PSF star, or later the QSO/host/companion system.

The residual flux in this process is of the order of 3% of the total, inside the 05 radius aperture where most apparent residuals are located, the absolute value of the residuals in the same region is 14%. This means that it will be generally impossible to detect any host galaxy of less than 3% of the total flux of the QSO, and it will even be difficult to isolate a somewhat brighter smooth galaxy in the non-smooth residuals. This level of residuals is consistent with experience from the HST ACS camera, where we find that due to PSF uncertainties 5% of the total flux are approximate detection limits for faint host galaxies [@jahn04b Jahnke et al. in prep.]. Including the structured PSF residuals we will only consider a host galaxy component as significant if it has clearly more than 3–5% of residual flux inside an 05 radius of the QSO, or that shows up as a non co-centric structure above the noise outside this region.

In absolute magnitudes and related to the QSO these limits correspond to the following: inside an 05 radius of the QSO we can hide a galaxy co-centric with the QSO of at least $M_H\sim-24.7$ (for the 3% case) or $M_H\sim-25.2$ (for 5%).

Results
=======

VISIR
-----

We detect a single unresolved point source in the VISIR field-of-view with a flux density 62.5 mJy at observed 11.3$\mu$m (Figure \[fig:allwave\], right column). This compares to 69.3 mJy in the IRAS 12 $\mu$m channel. There is no second source detected in the field down to a point source sensitivity of at least 3 mJy at the 5$\sigma$ level. Extended sources of the visual size of the companion galaxy have a 5$\sigma$ detection limit of 5.5 mJy.

With only one source in the total 192 VISIR field three optical sources have in principle to be considered as potential counterparts: The QSO nucleus, the companion galaxy, and the foreground star. However, the star is a G spectral type and can thus be safely ruled out.

We find that the initial position of the MIR point source as recorded in the VISIR image header comes to lie between the QSO and the companion, somewhat closer to the QSO. To clarify this we conducted an analysis of the pointing accuracy of VISIR testing the astrometry of a number of reference stars observed with VISIR at different epochs. The two results are: (1) In all cases the offset between targeted and effective RA,Dec is less than 1 rms, but (2) there is a systematic offset of 0.15s in RA recorded in the fits header, so the true positions need to be corrected by –0.15s in RA. This correction places the MIR point source exactly onto the locus of the QSO in the HST ACS images. It is thus clearly the QSO nucleus that is responsible for all of the 11.3 $\mu$m emission.

NICMOS
------

### Host galaxy {#results:host}

To extract information on the host galaxy, we use three different methods to remove the flux contribution from the QSO nucleus. First, we make a model-independent test for obvious extended emission: In a simple peak subtraction we remove a PSF from the QSO, scaled to the total flux inside two pixels radius around the QSO center. This is a robust approach that is independent of specific model assumptions and quite insensitive to the noise distribution in the image [@jahn04b]. As a result, the peak subtracted image shows no obvious extended residual, i.e. host galaxy, centered on the QSO, when using the PSF star as PSF.

As a second step we use on the one hand [galfit]{} to model the 2-dimensional light distribution of the [HE0450–2958]{} system and decompose it into different morphological components. On the other hand we use the MCS deconvolution method [@maga98] to mathematically deconvolve the system to a well defined and narrower PSF. The procedure we follow is based on the one described in @chan07. For [galfit]{} we use the two empirical PSFs, for MCS deconvolution we construct a number of combinations of empirical PSF and TinyTim models including very red dust-like SED components.

While these two approaches are complementary in method, their results agree as can be seen in Figure \[fig:dataimages\]: The inner part of the QSO inside of 05 radius is consistent with a point source within the PSF uncertainties, but there is extra flux present outside of this radius. The structure of the PSF removal or deconvolution residuals points to a substantial mismatch between shape of the QSO nucleus and the separately observed PSF star, but also to too simple models of TinyTim. In order to remove obvious residual PSF structure a very red SED needs to be assumed, which at this point can not be discriminated from a marginally resolved red component on top of the AGN point source. However, in light of the non-average properties of this QSO, a mean QSO SED is also not expected.

In the following we present our results in more detail and focus on the [galfit]{} results, since it allows a more direct estimate of the significance of detected structures. A comparison of the original and point source-removed images in the optical and NIR, and the MIR image are shown in Figure \[fig:allwave\].

We use [galfit]{} to perform a number of different model fits. In all of them the companion star and QSO nucleus are described by a pure point source, while the companion galaxy is fit with one or two Sérsic[^1] components with free axis ratio, or left unmodelled. We also attempt to add another Sérsic component for the putative host galaxy. We always leave the Sérsic parameter $n$ free, although the companion galaxy is too complex and the putative host galaxy too faint for $n$ to be interpreted physically.

With the PSF star used as PSF [galfit]{} finds a result consistent with the peak subtraction. A positive residual of $H\sim17.7$ inside a $\sim$1 radius aperture has a flux below 2% of the 13.05 mag of the QSO itself (see Figure \[fig:dataimages\]e+f). Even though we choose an aperture larger than in our calculation in Section \[sec:psfuncertainty\], we receive a value far below our significance limit, so no significant co-centered host galaxy is seen in this way.

If we use the foreground star as PSF (Fig. \[fig:dataimages\]g) we find – as expected – a residual flux that is slightly higher than before, and consistent values for two different approaches: First, for a pure PSF fit to the QSO location, integrating the flux of the residual within a 1 radius aperture around the QSO, except along the SE–NW-axis where we expect residual flux from foreground star and companion galaxy. Secondly, we get a similar flux for a fitted additional host galaxy Sérsic component.

These two approaches yield a magnitude of $H$$\sim$15.8 and 16.2, respectively, for the host, $\sim$1.5mag brighter than for the PSF star fit. $H$$\sim$16 corresponds to $\sim$6% of the 13.05 mag of the QSO nucleus.

Again the QSO residual shows substantial structure as reported in Section \[sec:psfuncertainty\]. It consists of nested rings of positive and negative flux, typical signs of a close but different width between the PSF we use and the actual one. The bulk of structure is contained in the innermost 05 radius and contains 2/3 of the residual flux. The remaining residual of 2% of the total flux outside this radius is again insignificant, and no main body of the host galaxy co-centered with the quasar is found which satisfies our significance criterion. Going back to the PSF residuals that we quantified earlier on, we detect no co-centered host galaxy at a level above 3% of the flux of the quasar nucleus, corresponding to an upper limit of $H=16.9$.

![image](fig3_all.eps){width="\textwidth"}

However, after removal of the point source, a feature becomes clearer, what we dub the “NE-extension”. This faint structure extends from the QSO to the N–E, and it can be traced starting at the edge of the strong PSF residuals at 06 (2.5 kpc) N–E of the nucleus (Figures \[fig:dataimages\] and \[fig:ne-extension\]). Some signs of it are already visible in the optical, when going back to the the F606W image [@maga05 see also Fig. \[fig:allwave\]], but it is much more pronounced in the new $H$-band data compared to the $V$-band. The NE-extension is possibly part of a tidal arm similar to the arm towards the south of the companion, already described by @cana01, but our $H$-band image shows it to be clearly disjoint from the companion galaxy. Due to its proximity it is very likely associated with the QSO, even though it is clearly not its main body. It is unlikely that the NE-extension is just a gas cloud with star formation induced by the radio jet in the system, since it lies at least 50$^\circ$ from the jet direction [@feai07]. It is also unlikely a chance superposition of a gas cloud with emission line gas, as seen by @leta08, since the observed $H$-band does not contain any strong enough line. The NE-extension contains non-negligible flux far above the noise of the background and is unaffected by QSO residuals and independent of the PSF used. We estimate its brightness at $H=18.8$ using an aperture encompassing all visible extension outside the QSO nucleus residual. The same region in the ACS image has $V=21.6$, so $(V-H)=2.8$

![\[fig:ne-extension\] A slight zoom into the inner region of [HE0450–2958]{} to show the newly found “NE-extension” of the QSO. We removed the star and the QSO using the PSF star as PSF. An extension to the N–E is visible (marked with [*red brackets*]{}) at a distance of 06–15 that is clearly not due to PSF residuals – a very similar result is seen when using the narrower foreground star as PSF (Fig. \[fig:dataimages\]e), or MCS deconvolution (Fig. \[fig:dataimages\]d). This structure is disjoint from the companion galaxy so very likely belongs to the QSO host galaxy itself. The estimated brightness is $H=18.8$. The image size is 45 on the side. ](fig4.eps){width="\columnwidth"}

In summary, we detect no significant host galaxy that is co-centered with the QSO. We conclude this from the size and shape of the residuals underneath the QSO in comparison to the “PSF star minus foreground star” subtraction residuals we discussed above. The NE-extension, however, that can be seen outside of the residuals of the QSO nucleus, is a real and significant emission structure – and it is very likely associated with the main part of the host galaxy.

### Companion galaxy

In the ACS $V$-band the companion galaxy located 15 to the S–E appears clumpy, with several bright knots as well as lower surface brightness in the center. @cana01 even call the companion a “collisional ring galaxy”. With the NICMOS $H$-band we get a substantially different picture. The galaxy at $H=15.2$ is still asymmetric, with tidal extensions, but contrary to the visible wavelengths it is smooth and shows a pronounced center: clear signs for substantial dust, distributed not smoothly but unevenly and clumpily, with concentration towards the center that only shows up in the optical (Fig. \[fig:companion\]). The complexity of the companion is manifested in that there is no good description with neither one or two Sersic components, when the azimuthal shape is restricted to ellipses. The Sersic index of the companion is around $n=2$ for a single Sersic component, and $n<1$ if two Sersic components are used. Taken at face value, both cases point to a more disk- than bulge-like companion, but a substantial fraction in flux is containted in the non-symmetric distorted part of the companion – and this should be the main description of the companion. More complex descriptions were put forward, with either a proposed additional faint AGN hosted by the companion galaxy [@leta09], explosive quasar outflows [@lipa09], or quasar-induced star formation [@elba09].

![\[fig:companion\] Zoom on the companion galaxy. As a difference to the $I$-band (Fig. \[fig:allwave\]) the galaxy has a pronounced peak of emission and no ring. The light in the optical is obviously attenuated by dust, very strongly in the center where the dust is optically thick, less in the outer regions. Image size is 3 on the side. ](fig5.eps){width="\columnwidth"}

Discussion
==========

Where is the ULIRG? {#sec:discussion_ulirg}
-------------------

There was substantial confusion about the source for the ULIRG-strength IRAS MIR and FIR emission in the literature. From the uncorrected \[OII\] line flux a star formation rate (SFR) of only 1 M$_\odot$/yr can be inferred [@kim07]. @maga05 still assign the ULIRG emission to the companion galaxy due to its Balmer decrement which yields non-negligible dust extinction, while @kim07 note that the corrected SFR would still be below 10 M$_\odot$/yr. This number is in strong disagreement with a SFR up to $\sim$800 M$_\odot$/yr inferred from total IR-luminosity or 370 M$_\odot$/yr from CO [@papa08].

The new NICMOS images show that the stars in the companion galaxy are not distributed in a ring, but smoothly (Fig. \[fig:companion\]) and that an optically thick dust creates the ring-like structure in the optical ACS images (Fig. \[fig:allwave\]a+b). This means that at optical wavelengths only information from the less extincted outer regions of the galaxy as well as the surface of the strongly extincted central regions is seen. UV-based SFRs must therefore dramatically underestimate the true SFRs when corrected with dust extinction estimated from (also optical wavelength) Balmer-decrements.

The actual scale of the uncertainty in $A_V$, the optical extinction correction, can be estimated by comparing $A_V$ estimates from Balmer lines and Paschen/Bracket lines in other ULIRGs. @dann05 studied five ULIRGS for which they estimated $A_V$ both from H$_\alpha$/H$_\beta$ as well as from Pa$\alpha$/Br$\gamma$. NIR-derived values for A$_V$ were in every case significantly larger, ranging from factors of $\sim$1.16 to $\sim$10 (mean 4.0) times higher. As this factor does not scale in any way with the optical $A_V$ estimate, but only with the NIR estimate, we can not determine a correction for [HE0450–2958]{}.

When starting out with the redshifted \[OII\]-line at $\lambda$3727 and $A_\mathrm{3727}=A_V\times1.57$ like @kim07, and the correction factors from @dann05, a huge range of possible star formation rates arises. An average of the two [*lowest*]{} correction values from @dann05 of 1.16 and 1.65 means $A_V\sim 2.1$, $A_\mathrm{3727}\sim3.3$ or corrected SFRs of 21 M$_\odot$/yr. Using their mean correction factor of $\sim$4 would lead to $A_\mathrm{3727}\sim9.4$ or $>$5000 M$_\odot$/yr. So already a number below the mean correction ($A_V\sim3$) would make these numbers consistent with FIR-emission based SFR estimates. This directly shows that optical/UV line-emission based SFRs as used by @kim07 can not at all be used to constrain the true SFR of ULIRGs and does not provide an argument against strong star formation in the companion.

@papa08 approximate the IRAS IR SED with a 2-component black-body model and find a cool component $T_\mathrm{dust}^\mathrm{cool}=47$K, dust mass $M_\mathrm{dust}^\mathrm{cool}\sim10^8$ M$_\odot$ and $L_\mathrm{FIR}
\sim 2.1\times10^{12}$ L$_\odot$, and a warm component with $T_\mathrm{dust}^\mathrm{warm}=184$K, $M_\mathrm{dust}^\mathrm{warm}\sim5\times10^4$ M$_\odot$ and $L_\mathrm{MIR} \sim 2.6\times10^{12}$ L$_\odot$. We can now for the first time spatially localize the warm component from the detection of the single 11.3$\mu$m point source with VISIR to be coincident with the position of the QSO nucleus. Since the measured flux density is consistent with a warm component having the previously known 12$\mu$m IRAS flux density, we conclude that the QSO nucleus itself already is a ULIRG-level emitter, but with a warmer component compared to star formation.

For localizing star formation in the system, there are two recent new datasets available, radio data from @feai07 and the CO maps by @papa08. While the radio maps do not set strong constraints when trying to exploit the radio–FIR relation to assign a location for the FIR emission, the CO data are more powerful: at least the bulk, possibly all of molecular gas and thus star formation activity is located in the companion galaxy.

We can add two further constraints from our NICMOS and VISIR images. Both the mid infrared SED of the system (Fig. \[fig:iras\_sed\]) as well as an extrapolation from the $H$-band are consistent with an Arp220-like star formation, while ruling out milder, M82-like conditions. In the latter case the companion would have to be visible in our observed 11.3$\mu$m image, but it is absent (Fig. \[fig:allwave\]). Together with the dense and clumpy dust geometry of the companion when comparing optical and NIR morphology, it becomes clear that the companion is responsible for most, if not all, of the 370 M$_\odot$/yr star formation.

If we follow the 5:1 CO detection significance for the companion given by @papa08, this means that as a minimum 5/6=83% of CO are located in the companion and thus also $\ge83$% of the star formation and FIR emission. This number converts to an integrated IR luminosity of $L_\mathrm{FIR} \ge 1.75\times10^{12}$ L$_\odot$, so the companion also qualifies as a ULIRG.

While the presence of very strong star formation in the companion is clear now, its trigger is a priori not so clear. The most probably solution is merger induced SF, so the system would be a classical ULIRG – just with a non-standard geometry – but there is room for a radio jet induced effect as well. One of the lobes of the jets from the QSO is located directly at the companion position. If and how much this contributes to star formation in the companion still needs to be quantified.

Host galaxy detection {#sec:hostgalaxydetection}
---------------------

With the companion identified as the main star-former, we get limits from the CO that less than 1/6 of the total cool dust is located within the putative host galaxy. Thus 1/6 of the FIR-inferred SFR by @papa08 of $\mathrm{SFR} = 1.76\cdot 10^{10} (\mathrm{L_{IR}/L_\odot})$ M$_\odot$/yr correspond to an upper limit of 62 M$_\odot$/yr. This leaves room for a non-negligible amount of SF in the host galaxy, but is also an upper limit[^2]. If we assume the host galaxy to have a mix of old and young stellar population as we find for other QSO host galaxies at these redshifts [@jahn04a; @leta07], we can convert this to an expected $H$-band flux. If the host galaxy had the same population mix as @cana01 modelled for the companion galaxy[^3] – 95.5% of a 10 Gyr old population with 5 Gyr e-folding SFR timescale plus 4.5% of a 128 Myr young population –, this SFR upper limit would translate to an expected NIR magnitude of 1.75 mag fainter than the companion or $H\ge16.95$. The combined color and $K$-correction term is $V-H_\mathrm{z=0.285} = 1.66$, and changes by only about $\pm$0.3mag for a pure old (10 Gyr) or young (100 Myr) population. So they are rather insensitive to the exact choice of stellar population. However, this limit will get brighter if the host galaxy contained less dust – by about 0.3 mag per magnitude decrease in $A_V$.

With that in mind, this limit is not more stringent than the limit from NICMOS itself: No significant main host galaxy body is found after PSF removal (Section \[results:host\]) and so an upper limit from the NIR decomposition of $H=16.9$ applies for a host galaxy co-centered with the quasar nucleus. We therefore conclude that the current upper limit from NICMOS lies at around $H\sim16.9$. This is consistent with the CO/FIR limits.

How do these numbers relate to the current upper limit for a co-centered host galaxy from the optical HST data? We convert our $H$-band limit to absolute $V$-band magnitudes with again the assumption of the host galaxy having the same stellar population mix as the companion. In the conversion to $M_V$ we assume two different values for dust extinction, (a) $A_V=0$, motivated by the nearly dust-free line of sight to the QSO nucleus, and (b) a moderate $A_V=1$ (corresponding to $A_{H\mathrm{(z=0.285)}}\sim0.29$). This yields host-galaxy upper limits of $M_V>-21.25$ and $>-22.55$, for the cases (a) and (b) respectively. If we convert the @maga05 upper limits to our $h=0.7$ cosmology and assume the same stellar population and dust properties, we receive $M_V>-20.6$ and $>-21.6$, respectively. We note here that this corresponds to a detection limit of only 1.5% of the total quasar flux in the optical. This factor of two is owed to the better determined PSF in the ACS images. This allows @maga05 to set somewhat stricter upper limits for a nucleus co-centered host galaxy component, particularly if a low dust extinction is present.

Concerning lower limits to the host galaxy, the NE-extension (Figure \[fig:ne-extension\]) is a structure of real emission that can be traced towards the QSO from $\sim$15 to a radius of 06, where the region of substantial PSF residuals begins. We can not say for sure whether it continues further inward from this position. Signs of this structure are visible in the ACS $V$-band (see Figure \[fig:allwave\], left column) but it is not clear whether the more compact region only $\sim$02 N–E of the nucleus in ACS image is real or an artefact of the deconvolution process. We measured the $(V-H)$-color to be 2.8 outside this region, which is consistent with a stellar population of intermediate age. In the dust-free case this color corresponds to a $\sim$2.1 Gyr old single stellar population [@bruz03 solar metallicity], for $A_V=1.0$ to an age of 800 Myr. This is consistent with stellar material from a host galaxy, e.g. tidally ejected disk stars.

We conclude that with its spatial detachment from the companion galaxy this NE-extension is likely a part of the host galaxy, possibly as a tidal extension, but its vicinity to the QSO makes other interpretations less likely. With this interpretation, we receive an $H\le18.8$ [*lower*]{} limit for the host, corresponding to $M_V<-20.4$ ($A_V=0$) or $<-20.7$ ($A_V=1$). If we include this off-center emission to the upper limit of a co-centered host galaxy, we obtain a total host galaxy upper limit of $M_V>-21.2$ and $-22.0$. We thus bracket the host galaxy luminosity in the $V$-band by 0.8 and 1.3 mag or factors of $\sim2$ and $\sim3.5$, respectively.

Formally, the CO detection significance and NICMOS give the same limit on a star formation rate of up to $\sim60$ M$_\odot$/yr. If we take into account the stricter ACS $V$-band limits of $M_V>-20.6$ and $>-21.6$, depending on dust cases (a) and (b), these are fainter by 1.3 and 0.6mag than the CO predicted magnitures. Inversely, these reduce the upper limits on star formation to 18 and 35 M$_\odot$/yr, respectively. Beyond $A_V=2$mag the CO and NICMOS limits again become the most stringent. This means that we can not rule out dust obscuration in the host galaxy. At the same time the dust-free line of sight to the quasar nucleus is a strong argument against large amounts of dust, unless a very special geometrical configuration is invoked, while the warm ULIRG emission from the QSO points to dust in the very central few 100 pc. Only better CO limits or a detection of the host galaxy in the NIR will be able to finally resolve this matter.

Black hole mass, galaxy luminosity, and the NLSy1 angle
-------------------------------------------------------

Black hole mass estimates for [HE0450–2958]{} vary significantly through the literature. The original 8$\times$10$^8$ $M_\odot$ [@maga05] were revised later to a substantially lower value of 4$\times$10$^7$ [@leta07]. Both values are virial estimates based on H$\beta$ width, but while narrow and broad components were separately measured in the former study, the FWHM of the whole line was used in the latter. This revised value is consistent with the independent virial estimate of 6–9$\times$10$^7$ by @merr06, and even with an estimate from X-ray variability, $2^{+7}_{-1.3}$$\times$10$^7$ [@zhou07]. Since the virial estimates agree now, we will adopt the range 4–9$\times$10$^7$ $M_\odot$ for the black hole mass.

@merr06 noted the rather narrow broad emission lines of [HE0450–2958]{} and suggested that it should actually be viewed not as a standard QSO but as a higher-$L$ analog of local NLSy1s. If we compare [HE0450–2958]{} with estimates from the literature [@grup04; @ohta07], we find that [HE0450–2958]{} is consistent with the high black hole mass end of the known NLSy1 distribution and does not need to constitute a new “higher-$L$ NLSy1 analog” class of its own. But is it consistent regarding other properties as well?

Morphologically, NLSy1 are mostly spirals, often barred, mostly not strongly disturbed [@ohta07]. Since galaxies have increasing bulge mass with increasing black hole mass it is not clear which structural properties to expect and if a merging system like this is consistent with the properties of the local, lower mass NLSy1 population.

There is even a debate on how different NLSy1 actually are from normal Seyferts. Recent studies show smaller BH mass differences between normal broad-line Sy1 and NLSy1 when using line dispersions instead of FWHM [@wats07], although a difference might remain. If galaxies with potentially core outflow-affected lines are considered separately, NLSy1 share the same $M_\mathrm{BH}-\sigma_\mathrm{bulge}$-relation with BLSy1, but their accretion rates are confirmed as lying often close to the Eddington limit [@komo07]. If we compute the [HE0450–2958]{} accretion rate – as derived from the $V$-band absolute magnitude of the quasar nucleus ($M_V=-25.75$, recomputed from the HST/ACS data with updated AGN color and $K$-correction) and a bolometric correction of $BC_V\sim8$ [@marc04; @elvi94] – in relation to its Eddington accretion rate, we obtain from $M_\mathrm{BH}=6.5\pm2.5\times10^7$ $M_\odot$ a super-Eddington accretion rate of $L/L_\mathrm{Edd}=6.2^{+3.8}_{-1.8}$. This is consistent with high Eddington ratios observed for NLSy1 [@warn04; @math05a].

![\[fig:m\_m\] $M_\mathrm{BH}$–$L_\mathrm{bulge}$-relation for inactive galaxies in the local Universe as presented by @tund07, with data from @haer04, @shan04 and @mclu04 [*(black lozenges and lines)*]{}. Overplotted are the upper limits for the host galaxy of [HE0450–2958]{} for the dust-free case by @maga05 from $V$-band imaging [*(small blue arrow)*]{} and with an $A_V=1$ added [*(small red arrow)*]{}, with their original black hole estimate, converted to our cosmology. The [*blue and red rectangles*]{} show the range for black hole mass estimates and our new lower limits for the (total) galaxy luminosity from NICMOS and new upper limits based on the (still better constrained) optical HST data. Note: Here we combined the off-center flux lower limit (NICMOS $H$-band) with the upper limit for a co-centered host galaxy (ACS $V$-band) for a total upper limit. The arrows to the bottom right show the conversion of our $L_\mathrm{galaxy}$ limits to $L_\mathrm{bulge}$ limits for bulge-to-disk ratios of 1:2 and 1:4. Both the dust-free as well as the $A_V=1$ dust case show a galaxy that is absolutely consistent with the black hole mass, even if the bulge-to-disk ratio is accounted for. ](fig6.eps){width="\columnwidth"}

With the new data and an explicit assumption/interpretation that the NE-extension is indeed associated with the host galaxy, we can for the first time present a black hole mass for [HE0450–2958]{} and bracketing limits for its host galaxy luminosity. We can thus place [HE0450–2958]{} on the $M_\mathrm{BH}$–$L_\mathrm{bulge}$-relation of active and inactive galaxies, with more than just an upper limit for galaxy luminosity. In Figure \[fig:m\_m\] we show data from @haer04 and others, as collected by @tund07. We overplotted the limits on [HE0450–2958]{} for the two assumptions of dust attenuation strength (Sec. \[sec:hostgalaxydetection\]). This shows that even when applying a sensible conversion factor of 1 to 1/4 (up to 1.5 mag) to convert from total to bulge luminosity, the host of [HE0450–2958]{} will be a perfectly normal galaxy in this parameter space, with a luminosity around the knee of the galaxy luminosity function, $L\sim L^*$.

Contrary to the claim by @maga05 it does not deviate substantially from the local $M_\mathrm{BH}$–$L_\mathrm{bulge}$-relation for normal inactive local massive galaxies, mainly due to the revised mass estimate for the black hole. However, this also means that [HE0450–2958]{} does not show a $M_\mathrm{BH}$/$L_\mathrm{bulge}$ different from local broad-line AGN, consistent with being a NLSy1-analog if the @komo07 result is taken as a base.

With the normal $M_\mathrm{BH}$/$L_\mathrm{bulge}$-ratio and the fact that we can now rule out huge amounts of obscuring dust around the QSO nucleus, the most likely explanation for the evasive host galaxy is indeed a high $L/L_\mathrm{Edd}$ accretion rate system – a NLSy1 at the high mass end of the normal NLSy1 population. With the current evidence Occam’s Razor favors this explanation over more exotic scenarios as the ejection of the QSO’s black hole in a 3-body interaction or a gravitational recoil event involving the companion galaxy [e.g. @hoff06; @haeh06; @merr06; @bonn07]. However, these scenarios are formally not ruled out even if the upper limit can be pushed down by another $\sim$5 magnitudes. All evidence combined is consistent with a system of a QSO with ULIRG-size IR emission, residing in an $L^*$ host galaxy that is in the process of colliding with a substantially more luminous and possibly more massive companion ULIR-galaxy[^4]. Much deeper high-resolution NIR imaging with a well controlled PSF are the best way to finally find and trace the here predicted host galaxy (bulge) component of [HE0450–2958]{} co-centered with the QSO nucleus and to estimate its luminosity and mass directly.

Black hole – galaxy coevolution
-------------------------------

Given the black hole mass and Eddington ratio the accretion rate of the BH is 1.4 M$_\odot$/yr. At the same time @papa08 derive a star formation rate from CO of 370 M$_\odot$/yr, predominantly in the companion galaxy. Applying a correction factor of 0.5 for mass returned to the interstellar matter by stellar winds, the stellar mass growth of the whole [HE0450–2958]{} system from star formation is 185 M$_\odot$/yr. The ratio of black hole accretion and stellar mass growth is then 12/185=6.5%, which is substantially higher than the $M_\mathrm{BH}$/$M_\mathrm{bulge}$ relation for local galaxies of 0.14% [@haer04].

We can conclude the following: If activity timescales are identical for star formation and BH accretion, this system grows in black hole mass much more rapidly than the bulge is required to grow to keep the system on the $M_\mathrm{BH}$/$M_\mathrm{bulge}$ relation. This is not possible, since the star formation is taking place in the companion and not the host galaxy. So in any case a potential maintainance of the relation for this system, if actually true, needs to be seen as an integral over more than several 10$^8$ yrs.

On the other hand, a gas consumption timescale of 9.5$\times$10$^7$ yrs – if we divide the H$_2$ masses and SF rates derived by @papa08 and account for 50% mass recycling – is possibly longer than the luminous quasar accretion phase. This would add to the requirement, that processes like the tidal forces of the galaxy interaction redistribute mass, adding stars to the bulge of the host galaxy. These were to the larger extent already preexisting in the host galaxies disk or the companion before the interaction and not created only now. The “coevolution” of the host galaxy and its black hole in [HE0450–2958]{} is clearly a two-part process: the build-up of stellar mass and the build-up of black hole and bulge mass. The former will take place on timescales of $>$1 Gyr through star formation, the latter two can “coevolve” if seen as an average over timescales of longer than the BH accretion lifetime, and a few dynamical timescales for redistribution of stellar orbits of, say, $<$500 Myrs.

How many [HE0450–2958]{}s are there?
------------------------------------

[HE0450–2958]{} is an unusual object. AGN in ULIRGs are common, but AGN right next to ULIRGs are not, particularly not luminous QSOs with inconspicuous host galaxies next to extreme starformers. So is [HE0450–2958]{} one of a kind or was it just the scarceness of IR imaging with 1 resolution and high-resolution CO maps that prevents us from finding similar objects en masse?

In the higher redshift Universe there was a recent report of a very similar system [@youn08]. LH850.02 at $z=3.3$ is the brightest submm galaxy in the Lockman hole. Using the Submillimeter Array, the authors find two components of which one is a ULIRG with intense star formation, while the other component likely harbors an AGN. At $z>2$ however, objects like this might be quite common, since merging rates and gas reservoirs were much larger than today. If there existed a substantial number of similar systems at low redshifts, this would allow to study mechanisms of the high-redshift Universe at much lower distances.

We try to estimate the frequency of such systems in the local Universe using the three morphologically best studied samples of quasars at $0.05\la z<0.43$. We deliberately use optically selected quasars only, as they have no bias with respect to frequency of merger signatures or extreme SFRs as IR-selected samples have by construction. In this way statements about the general population are possible. @jahn04a investigated a volume-limited and complete sample of 19 luminous QSOs out to $z=0.2$. While at least five of these QSOs are seen in intermediate and late stages of major mergers, only one, HE1254–0934, is a likely ULIRG[^5], as determined from its IRAS fluxes. It is also among the most distorted systems, with a companion at $\sim$1 distance from the QSO nucleus. The companion is more luminous than the host galaxy, and shows a substantial tidal tail. It looks remarkably similar to [HE0450–2958]{}.

The two other samples are not volume-limited samples, so the selection function is unclear – except that these quasars stem from either optical or radio surveys, but not the IR. @floy04 studied the morphologies of two intermediate- and high-luminosity samples of ten radio-quiet and seven radio-loud quasars at $0.29<z<0.43$, using HST-imaging data. Only one of their 17 quasars shows a distorted geometry similar to [HE0450–2958]{}(1237–040 at $z=0.371$) but there exists no information about the total IR emission or star-formation rates. The IRAS flux limits of 200mJy is equivalent to upper limits of $L_\mathrm{ir}\sim6\times10^{12}$ $L_\odot$ at $z=0.37$. ULIRG-strength emission for 1237–040 could have gone unnoticed by IRAS.

A recent study by @kim08b determined the morphologies of 45 HST-archived quasars at $z<0.35$. It has one object in common with @floy04 and three objects with @jahn04a. Of their sample, three other objects (HE0354–5500, PG1613+658, PKS2349–01) are clearly merging with a nearby companion, and are likely ULIRGs as judged from their IRAS fluxes. However, only in the case of HE0354–5500 the quasar and companion are still well separated and their envelopes have not yet merged into a common halo. The two other cases are in a very late merger state and star-formation will likely occur all over the system.

This adds up to only $\le$3/77 QSOs to possibly be [HE0450–2958]{}-like in the three samples combined. At $\le 4$% such systems are indeed rare in the local Universe. These three quasars however should be investigated in more detail. It needs to be tested how strong their star-formation actually is, where in the system it is localized, and if the separated companion is in any way connected to the AGN-fuelling. If a similar situation as for [HE0450–2958]{}is found, the result can set strong constraints on the ULIRG–AGN evolutionary scenario [@sand96] and the creation mechanisms of AGN at high redshifts. It can contribute to answering the question whether SF-ULIRG activity in AGN systems is an indicator of a specific mechanism of AGN fuelling. Or, if these are just the most gas-rich merger-triggered AGN systems at the top end of SFRs, with a continuous sequence towards less gas-rich merger-triggered AGN systems. The merging–AGN fuelling mechanism could be identical from ULIRGs down to the Seyfert regime, where at some point secular mechanisms become more dominant. Lower SFR systems could just be the consequence of lower gas mass, but this might only mildly impact on the – much smaller – AGN fuelling rate.

Conclusions
===========

With new NIR and MIR images to spatially resolve the [HE0450–2958]{} system, and in the light of previously existing data, we find:

1.  The companion galaxy is covered in optically thick and unevenly distributed dust. This makes it appear as a collisional ring galaxy in the optical, but intrinsically it is smooth and has smooth NIR emission increasing towards a pronounced center. The star formation in the companion is similar to the strong starburst Arp220, while softer M82-like star formation is ruled out. This can reconcile the SFR estimates from the optical and FIR. The companion is a star-formation powered ULIRG.

2.  Our MIR image confirms a single warm dust point source at the location of the QSO nucleus. This supports a two component dust SED with the warm component fully associated with the QSO nucleus, which is an AGN-powered ULIRG.

3.  A dust-free line of sight to the quasar nucleus is evidence that the host galaxy is not obscured by large amounts of dust. However, the ULIRG-strength warm IR emission by the nucleus and the upper limit on star formation in the host galaxy of substantial 60 M$_\odot$/yr leave room for dust.

4.  With $H\ge16.9$ the current NICMOS images do not set stronger upper limits on the host galaxy of [HE0450–2958]{}. The $V$-band, $H$-band, and CO-constraints give $M_V\ge-21.2$ to $M_V\ge-22.0$ depending on the assumed dust masses.

5.  Flux in the NE-extension of $H=18.8$ is likely associated with the QSO’s host galaxy. It corresponds to a first lower limit of $M_V<-20.4$ for the host galaxy. With a black hole of $\sim6.5\pm 2.5
     \times10^7$ M$_\odot$, an accreting rate of 12 M$_\odot$/yr equal to super-Eddington accretion, $L/L_\mathrm{Edd}=6.2^{+3.8}_{-1.8}$, the host galaxy is consistent with the $M_\mathrm{BH}$–$M_\mathrm{bulge}$-relation for normal galaxies. It is also consistent with [HE0450–2958]{} being a NLSy1 at the high end of the known black hole mass distribution. The reason for the high accretion rate is unclear but could be connected to [HE0450–2958]{} being in an early stage of merging with its gas-rich companion. A more exotic explanation for the system is currently not required by any data, but can in the end only be ruled out with much deeper, high-resolution NIR images to find the main body and bulge of the host galaxy.

6.  If host galaxy and black hole in [HE0450–2958]{} are co-evolving according to the local $M_\mathrm{BH}$–$M_\mathrm{bulge}$ relation, it has to occur over longer timescales ($\le$500 Myr) and/or the mass growth for the bulge is predominantly not caused by the current star formation in the system, but by redistribution of preexisting stars.

7.  A constellation as in the [HE0450–2958]{} system with separate locations of QSO nucleus and strongly star forming ULIRG companion might be common at $z>2$ where gas masses and merger rates were higher, but at a fraction of $\le$4% it is extremely rare in the local Universe.

The authors would like to thank E. F. Bell, A. Martínez Sansigre, H.Dannerbauer, E. Schinnerer, K. Meisenheimer, F. Courbin, P. Magain and H.-R. Klöckner for very fruitful discussions and helpful pointers.

Based on observations made with ESO Telescopes at the Paranal Observatory under programme ID 276.B-5011. Also based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program \#10797. This research has made use of the NASA/IPAC Extragalactic Database (NED).

KJ acknowledges support through the Emmy Noether Programme of the German Science Foundation (DFG) with grant number JA 1114/3-1. AB is funded by the Deutsches Zentrum für Luft- und Raumfahrt (DLR) under grant 50 OR 0404. VC, Research Fellow, thanks Belgian Funds for Scientific Research. This work was also supported by PRODEX experiment arrangement 90312 (ESA and PPS Science Policy, Belgium).

[*Facilities:*]{} , .

[54]{} natexlab\#1[\#1]{}

, J. N., [Kirhakos]{}, S., & [Schneider]{}, D. P. 1994, ApJ, 435, L11

Bahcall, J. N., Kirhakos, S., & Schneider, D. P. 1995, ApJ, 450, 486

, A., [Cox]{}, P., [Benford]{}, D. J., [Dowell]{}, C. D., [Kov[á]{}cs]{}, A., [Bertoldi]{}, F., [Omont]{}, A., & [Carilli]{}, C. L. 2006, ApJ, 642, 694

, E. W., [Shields]{}, G. A., & [Salviander]{}, S. 2007, ApJ, 666, L13

, P. J., [Disney]{}, M. J., [Blades]{}, J. C., [Boksenberg]{}, A., [Crane]{}, P., [Deharveng]{}, J. M., [Macchetto]{}, F. D., [Mackay]{}, C. D., & [Sparks]{}, W. B. 1996, ApJ, 473, 760

, G., & [Charlot]{}, S. 2003, MNRAS, 344, 1000

, G., & [Stockton]{}, A. 2001, ApJ, 555, 719

, V., & [Magain]{}, P. 2007, A&A, 470, 467

, H., [Rigopoulou]{}, D., [Lutz]{}, D., [Genzel]{}, R., [Sturm]{}, E., & [Moorwood]{}, A. F. M. 2005, A&A, 441, 999

, K. M., [Tacconi]{}, L. J., [Davies]{}, R. I., [Genzel]{}, R., [Lutz]{}, D., [Naab]{}, T., [Burkert]{}, A., [Veilleux]{}, S., & [Sanders]{}, D. B. 2006, ApJ, 638, 745

, M. H. K., [Lub]{}, J., & [Miley]{}, G. K. 1987, AApSS, 70, 95

, D., [Cesarsky]{}, C. J., [Chanial]{}, P., [Aussel]{}, H., [Franceschini]{}, A., [Fadda]{}, D., & [Chary]{}, R. R. 2002, A&A, 384, 848

, D., [et al.]{} 2009, to be submitted to A&A

Elvis, M., Wilkes, B. J., McDowell, J. C., Green, R. F., Bechtold, J., Willner, S. P., Oey, M. S., Polomski, E., & Cutri, R. 1994, ApJS, 95, 1

, I. J., [Papadopoulos]{}, P. P., [Ekers]{}, R. D., & [Middelberg]{}, E. 2007, ApJ, 662, 872

, D. J. E., [Kukula]{}, M. J., [Dunlop]{}, J. S., [McLure]{}, R. J., [Miller]{}, L., [Percival]{}, W. J., [Baum]{}, S. A., & [O’Dea]{}, C. P. 2004, MNRAS, 355, 196

, M. A. T., [Girardi]{}, L., [Hatziminaoglou]{}, E., [Benoist]{}, C., [Olsen]{}, L. F., [da Costa]{}, L., [Arnouts]{}, S., [Madejsky]{}, R., [Mignani]{}, R. P., [Rit[é]{}]{}, C., [Sikkema]{}, G., [Slijkhuis]{}, R., & [Vandame]{}, B. 2002, A&A, 392, 741

, D., & [Mathur]{}, S. 2004, ApJ, 606, L41

, M. G., [Davies]{}, M. B., & [Rees]{}, M. J. 2006, MNRAS, 366, L22

, N., & [Rix]{}, H.-W. 2004, ApJ, 604, L89

, L., & [Loeb]{}, A. 2006, ApJ, 638, L75

, P. F., [Hernquist]{}, L., [Cox]{}, T. J., [Di Matteo]{}, T., [Robertson]{}, B., & [Springel]{}, V. 2006, ApJS, 163, 1

Jahnke, K., Kuhlbrodt, B., & Wisotzki, L. 2004, MNRAS, 352, 399

Jahnke, K., Sánchez, S. F., Wisotzki, L., Barden, M., Beckwith, S. V. W., Bell, E. F., Borch, A., Caldwell, J. A. R., Häu[ß]{}ler, B., Heymans, C., Jogee, S., McIntosh, D. H., Meisenheimer, K., Peng, C. Y., Rix, H.-W., Somerville, R. S., & Wolf, C. 2004, ApJ, 614, 568

, M., [Ho]{}, L. C., [Peng]{}, C. Y., [Barth]{}, A. J., [Im]{}, M., [Martini]{}, P., & [Nelson]{}, C. H. 2008, submitted to ApJ, arxiv/0807.1337

, M., [Ho]{}, L. C., [Peng]{}, C. Y., & [Im]{}, M. 2007, ApJ, 658, 107

, S., & [Xu]{}, D. 2007, ApJ, 667, L33

Krist, J. 2003, STSCI, http://www.stsci.edu/software/tinytim

, G., [Magain]{}, P., [Chantry]{}, V., & [Letawe]{}, Y. 2009, MNRAS, 607

, G., [Magain]{}, P., & [Courbin]{}, F. 2008, A&A, 480, 69

, G., [Magain]{}, P., [Courbin]{}, F., [Jablonka]{}, P., [Jahnke]{}, K., [Meylan]{}, G., & [Wisotzki]{}, L. 2007, MNRAS, 378, 83

, S., [Bergmann]{}, M., [Sanchez]{}, S. F., [Garcia-Lorenzo]{}, B., [Terlevich]{}, R., [Mediavilla]{}, E., [Taniguchi]{}, Y., [Zheng]{}, W., [Punsly]{}, B., [Ahumada]{}, A., & [Merlo]{}, D. 2009, ApJ (in press), arXiv/0901.3292

, F. J., [Cutri]{}, R. M., [Huchra]{}, J. P., & [Kleinmann]{}, S. G. 1988, ApJ, 327, L41

, F. J., [Cutri]{}, R. M., [Kleinmann]{}, S. G., & [Huchra]{}, J. P. 1989, ApJ, 340, L1

Magain, P., Courbin, F., & Sohy, S. 1998, ApJ, 494, 472

, P., [Letawe]{}, G., [Courbin]{}, F., [Jablonka]{}, P., [Jahnke]{}, K., [Meylan]{}, G., & [Wisotzki]{}, L. 2005, Nature, 437, 381

, A., [Risaliti]{}, G., [Gilli]{}, R., [Hunt]{}, L. K., [Maiolino]{}, R., & [Salvati]{}, M. 2004, MNRAS, 351, 169

, S., & [Grupe]{}, D. 2005, ApJ, 633, 688

McLeod, K. K., & Rieke, G. H. 1995, ApJ, 454, L77

, R. J., & [Dunlop]{}, J. S. 2004, MNRAS, 352, 1390

, D., [Storchi-Bergmann]{}, T., [Robinson]{}, A., [Batcheldor]{}, D., [Axon]{}, D., & [Cid Fernandes]{}, R. 2006, MNRAS, 367, 1746

, K., [Aoki]{}, K., [Kawaguchi]{}, T., & [Kiuchi]{}, G. 2007, ApJS, 169, 1

Pantin, E., Vanzi, L., & Weilenman, U. 2007, in 2007 ESO Instrument Calibration Workshop (Springer Verlag)

, P. P., [Feain]{}, I. J., [Wagg]{}, J., & [Wilner]{}, D. J. 2008, ApJ, 684, 845

Peng, C. Y., Ho, L. C., Impey, C. D., & Rix, H.-W. 2002, AJ, 124, 266

Sanders, D. B., & Mirabel, I. F. 1996, ARA&A, 34, 749

Sérsic, J. 1968, Atlas de Galaxias Australes, Observatorio Astronomico de Cordoba

, F., [Salucci]{}, P., [Granato]{}, G. L., [De Zotti]{}, G., & [Danese]{}, L. 2004, MNRAS, 354, 1020

, R. S., [Hopkins]{}, P. F., [Cox]{}, T. J., [Robertson]{}, B. E., & [Hernquist]{}, L. 2008, MNRAS, 391, 481

, E., [Bernardi]{}, M., [Hyde]{}, J. B., [Sheth]{}, R. K., & [Pizzella]{}, A. 2007, ApJ, 663, 53

, C., [Hamann]{}, F., & [Dietrich]{}, M. 2004, ApJ, 608, 136

, L. C., [Mathur]{}, S., & [Grupe]{}, D. 2007, AJ, 133, 2435

, J. D., [Dunlop]{}, J. S., [Peck]{}, A. B., [Ivison]{}, R. J., [Biggs]{}, A. D., [Chapin]{}, E. L., [Clements]{}, D. L., [Dye]{}, S., [Greve]{}, T. R., [Hughes]{}, D. H., [Iono]{}, D., [Smail]{}, I., [Krips]{}, M., [Petitpas]{}, G. R., [Wilner]{}, D., [Schael]{}, A. M., & [Wilson]{}, C. D. 2008, MNRAS, 387, 707

, X.-L., [Yang]{}, F., [L[ü]{}]{}, X.-R., & [Wang]{}, J.-M. 2007, AJ, 133, 432

[^1]: The Sérsic profile [@sers68] is a generalized galaxy profile with variable wing strength, set by the “Sérsic-parameter” $n$. It reverts to an exponential disk profile typical for spiral galaxies for $n=1$ and for $n=4$ it becomes a de Vaucouleurs profile found for many elliptical galaxies.

[^2]: Note that for the galaxy-scale star formation regions around QSO nuclei the dust can be heated by a mix of stellar emission as well as energy from the AGN. In this sense the 47 K found for the cool dust component of [HE0450–2958]{} agrees well with the mean SF-heated dust around higher-$z$ QSOs [also 47 K, @beel06], and can be composed of intrisically cooler dust (20–30 K) plus AGN heating. This temperature could thus be a hint that indeed a part of this cool dust component is located in the QSO host galaxy and not in the companion.

[^3]: @cana01 used optical spectra only. With the optically thick dust now detected we have to restrict their diagnosis to mainly the outer parts and surface of the companion. The population mix there might be identical to the core of the companion, but it does not necessarily have to.

[^4]: It is interesting to note that the “companion” is close to a factor of 10 more luminous than the host galaxy. With all uncertainties included it would still appear as if the typical mass ratio upper limit of 1:3 for the merging galaxies in a ULIRG system [@dasy06] were exceeded here. However, when using the dynamical masses from @papa08 to predict a black hole mass in the host galaxy consistent with the @haer04 relation, we get a merger mass ratio of 1:1 or 1:2.

[^5]: This is a borderline case because it will fall slightly below or above the ULIRG definition limit depending on if we include upper limits in 12 and 25$\mu$m or assume the flux to be zero.
