---
author:
- 'K. Sandstrom'
- 'O. Krause'
- 'H. Linz'
- 'E. Schinnerer'
- 'G. Dumas'
- 'S. Meidt'
- 'H.-W. Rix'
- 'M. Sauvage'
- 'F. Walter'
- 'R. C. Kennicutt'
- 'D. Calzetti'
- 'P. Appleton'
- 'L. Armus'
- 'P. Beirão'
- 'A. Bolatto'
- 'B. Brandl'
- 'A. Crocker'
- 'K. Croxall'
- 'D. Dale'
- 'B. T. Draine'
- 'C. Engelbracht'
- 'A. Gil de Paz'
- 'K. Gordon'
- 'B. Groves'
- 'C.-N. Hao'
- 'G. Helou'
- 'J. Hinz'
- 'L. Hunt'
- 'B. D. Johnson'
- 'J. Koda'
- 'A. Leroy'
- 'E. J. Murphy'
- 'N. Rahman'
- 'H. Roussel'
- 'R. Skibba'
- 'J.-D. Smith'
- 'S. Srinivasan'
- 'L. Vigroux'
- 'B. E. Warren'
- 'C. D. Wilson'
- 'M. Wolfire'
- 'S. Zibetti'
title: 'Mapping far-IR emission from the central kiloparsec of [NGC$\,1097$]{}[^1]'
---

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

The central regions of galaxies host some of the most intense star-formation that we can observe in the local Universe in circumnuclear starburst rings. Starburst rings are believed to be the consequence of the pile-up of inflowing gas and dust, driven by a non-axisymmetric potential from a stellar bar, on orbits located near the Inner Lindblad Resonance of the bar [@combes85; @athanassoula92]. The high surface densities that exist in the ring lead to high star-formation rates. Indeed starburst rings are one of few regions in non-interacting galaxies where the formation of “super star clusters” commonly occurs [@maoz96]. The stars formed in the ring can be numerous enough to drive the structural evolution of the galaxy [@norman96; @kormendy04] and can be the dominant power source for the galaxy’s infrared (IR) emission.

Star-formation in circumnuclear rings occurs under conditions not normally found in the disks of galaxies: in addition to their high gas surface densities, these regions have dynamical timescales that are comparable to the lifetimes of massive stars. Understanding star formation in circumnuclear rings has been a long-standing problem [@combes96]. There are two main models: the “popcorn” model [@elmegreen94], where star-formation is driven by stochastic gravitational fragmentation along the ring, and the “pearls on a string” model, where gas flowing into the ring is compressed near the contact points (i.e. locations where the dust lanes intersect the ring) and then forms stars a short distance downstream [e.g., @boker08]. The “pearls on a string” model predicts a gradient in the ages of young stellar clusters as one moves away from the contact points. This has been observed in a number of starburst rings [e.g., @mazzuca08; @boker08]. Conversely, many well-studied rings show no evidence for an age gradient [@maoz01]. It is not obvious, however, that a single mode of star-formation must occur in all rings or even at all times in a given ring [@vandeven09].

KINGFISH (Key Insights into Nearby Galaxies: A Far-Infrared Survey with [*Herschel*]{}, PI R. Kennicutt) is an Open-Time Key Program to study the interstellar medium (ISM) of nearby galaxies with far-IR/sub-mm photometry and spectroscopy. Among the unique aspects of the KINGFISH science program is the ability to observe thermal dust emission at unprecedented spatial resolution ($\sim$ 5.6, 6.8 and 11.3at 70, 100 and 160 [$\mu$m]{}) using PACS (Photodetector Array Camera and Spectrometer) imaging. High spatial resolution is crucial for observing processes occurring in the central regions of galaxies. These regions represent our best opportunity to study in detail the interplay between dynamics, star-formation and feedback that regulate the fueling of nuclear activity, be it a starburst or an active galactic nucleus (AGN).

Below we present PACS imaging of the galaxy NGC 1097, one of the first KINGFISH targets observed during the [*Herschel*]{} Science Demonstration Program (SDP) (for PACS spectroscopy of NGC 1097 see Beirão et al. 2010 and for [*SPIRE*]{} observations see Engelbracht et al. 2010). The source NGC 1097 is a barred spiral galaxy located at a distance of 19.1 Mpc [@willick97 1$\approx 92$ pc]. In its central kpc it hosts an intensely star-forming [$\sim 5$  yr$^{-1}$; @hummel87] ring with a radius of $\sim 900$ pc. The ring’s rotation speed of $\sim 300$ km s$^{-1}$ [corrected for inclination, @storchi-bergmann96], corresponds to a rotation period of $\sim 18$ Myr. The galaxy’s nucleus is classified as a LINER from optical emission line diagnostics [@phillips84], but is shown to be a Seyfert 1 by its double-peaked H$\alpha$ profile [@storchi-bergmann93]. UV spectroscopy has revealed a few Myr old burst of star-formation in the central 9 pc of the galaxy [@storchi-bergmann05]. With the high spatial resolution of [*Herschel*]{} PACS, we can resolve the starburst ring and inner 600 pc of NGC 1097 for the first time at wavelengths near the peak of the dust spectral energy distribution (SED).

Observations and data reduction
===============================

The galaxy NGC 1097 was observed with the PACS instrument (Poglitsch et al. 2010) on the [*Herschel*]{} Space Observatory (Pilbratt et al. 2010) on 2009 December 20 during the SDP. We obtained 15 long scan-maps in two orthogonal directions at the medium scan speed (20s$^{-1}$). The scan position angles (45$^\circ$ relative to the scan direction) provide homogeneous coverage over the mapped region. The total on-source times per pixel were approximately 150, 150, and 300 seconds for 70, 100 and 160 [$\mu$m]{}, respectively.

The raw data were reduced with HIPE (Ott 2010), version 3.0, build 455. Besides the standard steps leading to level-1 calibrated data, second-level deglitching and correction for offsets in the detector sub-matrices were performed. The data were then highpass-filtered using a median window of 5 to remove the effects of bolometer sensitivity drifts and 1/f noise. We masked out emission structures (visible in a first iteration of the map-making) with a 5-wide mask before computing this running median to prevent subtraction of source emission. Although the filtering may remove some extended flux from the galaxy, because we are primarily interested in the very bright central 1 of NGC 1097 this effect will be negligible. Finally, the data were projected onto a coordinate grid with 1 pixels.

After pipeline processing we applied flux correction factors from the PACS team to adjust the calibration. The current calibration has uncertainties of $\sim 10$, 10, and 20% for the 70, 100 and 160 [$\mu$m]{} bands, respectively (Poglitsch et al. 2010). Because we aim to compare our PACS observations with ancillary data at other wavelengths, we adjusted the relative astrometry of the PACS observations to match that of the [*Spitzer*]{} 24 [$\mu$m]{} from SINGS [Spitzer Infrared Nearby Galaxies Survey: @kennicutt03]. This was done by measuring the positions of background point-sources in the MIPS 24 [$\mu$m]{} (Multi-Band Imaging Photometer) and PACS 100 [$\mu$m]{} images, adjusting the PACS 100 [$\mu$m]{} astrometry, assuming the relative astrometry for the PACS bands is well-calibrated and transferring the solution to the other bands. The offset between the PACS and MIPS astrometry was $\sim 2$. The one-sigma surface brightness sensitivities per pixel of the final maps are 5.9, 6.2 and 3.3 MJy sr$^{-1}$. In Fig \[fig:rgb\] we show the three PACS images with a logarithmic stretch to highlight the spiral arms. Note that below we extract photometry from the images at their native resolution using apertures larger than the beam size of the lowest resolution map.

[*Herschel* PACS observations of the circumnuclear ring in NGC 1097]{}
======================================================================

The most prominent far-IR structure in NGC 1097 is its circumnuclear starburst ring, shown in Fig \[fig:ring\] at a variety of wavelengths. The PACS angular resolution allows us to clearly separate the contribution of the ring and nucleus from the galaxy’s emission for the first time at wavelengths that probe the peak of the dust SED. Summing the emission within a radius of 20(1.8 kpc) of the center and comparing it with the total flux from the galaxy, we find that the ring and nucleus emit 75, 60 and 55% of the total flux of NGC 1097 at 70, 100 and 160 [$\mu$m]{}, respectively (there is some galactic emission within 1.8 kpc that is not associated with the ring or nucleus, but this component is negligible). These measurements imply that the SED of the more extended galactic emission peaks at longer wavelengths than the SED of the ring. Indeed, by fitting a modified blackbody to the MIPS and [*SPIRE*]{} photometry of the galaxy, Engelbracht et al. (2010) find that the central region of NGC 1097 is 22% warmer than the disk.

The mid- and far-IR images of the ring in Fig \[fig:ring\] show similar structures. The ring is continuous (i.e. no obvious gaps) with a series of bright knots. The same knots are visible in each PACS image, although at 160 [$\mu$m]{} they are not well-resolved. At 70 and 100 [$\mu$m]{}, the surface brightness of the ring varies by less than $\pm 15$% about the mean on 600 pc scales. The variations at 24 [$\mu$m]{} on the same spatial scales are $\pm 25$%. The similarities from mid- to far-IR suggest that dust temperatures are not varying substantially in the ring, which we quantify below.

It is interesting to note that the same pattern of bright knots is not observed in carbon monoxide (CO) (shown in panel h of Fig \[fig:ring\]) or other dense gas tracers at comparable resolution [@kohno03; @hsieh08]. Instead the CO intensity peaks near the contact points and is much fainter over the rest of the ring. The differences between the far-IR and CO emission may be due to different CO excitation mechanisms in the shocked gas near the contact points or by the consumption and/or dissociation of molecular gas by star-formation events shortly downstream from the contact points. Three of the bright knots are also prominent in 3.5 cm radio continuum (as shown in panel f of Fig \[fig:ring\]). @beck05 showed that the radio knots have a flatter radio spectral index than the rest of the ring, most likely due to either a contribution from free-free emission from H II regions or synchrotron emission from young supernova remnants (SNRs), which has an intrinsically flatter spectrum. Because young SNRs will heat only a small fraction of the dust, an enhancement of thermal radio continuum and dust heating in and around H II regions may be the best explanation for the origin of the coincident bright radio and far-IR knots.

In Fig \[fig:randaz\] we show the mid- and far-IR band ratios as a function of azimuthal angle. The surface brightness was measured in 9 azimuthal bins with inner and outer radii of 5 and 15 to adequately sample the PSF out to 100 [$\mu$m]{}. The largest variations are in the 24/70 ratio, which peaks shortly downstream from the northernmost contact point, and varies by $\pm 15$%. The 70/100 ratio varies by less than $\pm 5$% around the ring. If there is a well-defined age gradient along the ring as predicted by the “pearls on a string” model, one might expect a gradient in dust temperatures moving away from the contact points. In the youngest star-forming regions, the radiation field will be more intense due to the presence of the most massive and short-lived stars and the regions will be more compact. Both of these effects lead to hotter dust temperatures. For instance, @groves08 modeled the spectra of the H II regions plus surrounding photo-dissociation region for star clusters with ages between 0.1$-$10 Myr. They find that for a cluster mass of $\sim
10^5$  and an ISM pressure of P/k $\sim 10^6$ K cm$^{-3}$ [approximately what has been deduced for the ISM in the circumnuclear ring by @hsieh08], the 24/70 and 70/100 band ratios decrease by 90 and 70% (factors of $\sim 7$ and 3, respectively) as the cluster ages from 1 to 10 Myr. The @groves08 models represent an upper bound to the band ratio variation we could expect if the ring was comprised solely of a well-defined sequence of aging clusters between 1$-$10 Myr old.

That we do not see large mid- and far-IR band ratio gradients does not rule out the existence of “pearls on a string” in favor of “popcorn” in NGC 1097, however. It may be the case that dust in the ring is predominantly heated by the radiation field from older stars (e.g. B stars with lifetimes of 10-100 Myr), which are uniformly distributed around the ring after a number of revolutions. Stellar population studies of the central kpc of NGC 1097 have shown that intermediate age stars make a considerable contribution to the UV radiation field in the vicinity of the ring [e.g. @bonatto98]. In this situation, the variation due to an age gradient would be diluted depending on the relative contribution of young clusters to the total dust heating. In addition, one might expect that given the fast dynamical time in the ring that local enhancements of dust heating are quickly wiped out. Stars and interstellar matter in NGC 1097’s ring traverses the distance between the two contact points in $\sim 9$ Myr. Even assuming that the cluster formation happens instantaneously after entering the ring, there will still be an abundance of massive stars by the time the cluster crosses half the ring.

Limits on the nuclear flux in the far-IR
========================================

High resolution imaging at mid- and near-IR wavelengths of the nucleus of NGC 1097 shows an unresolved central point source [@prieto05; @mason07] which contains a nuclear starburst [@storchi-bergmann05] and the AGN. No observations can yet resolve the AGN or central starburst, but it is still possible to distinguish the contributions of the different sources to some degree. @mason07, for instance, found that the 12 and 18 [$\mu$m]{}emission arises primarily from dust heated by the nuclear starburst rather than the AGN torus. In NGC 1097, previous far-IR flux limits for the nucleus were dominated by the starburst ring and provide limited information about the nuclear starburst or the AGN. With our [*Herschel*]{} observations, we can place limits on the flux arising in the central $\sim 600$ pc. We use the PACS PSF observations of Vesta with a 20s$^{-1}$ scan speed scaled to match the peak intensity of the point source we see in the center. Our scaled-PSF photometry is possible at 70 and 100 [$\mu$m]{}, but not at 160 [$\mu$m]{}where the central source is not well-resolved. The best scaled PSF has a total flux of 3.5 and 7.3 Jy at 70 and 100 [$\mu$m]{}. Without more detailed modeling of the nuclear region these measurements should only be considered upper limits. However, they improve constraints on the nuclear flux by more than an order of magnitude as shown in Fig \[fig:sed\], which presents the SED of the nucleus from a compilation by @prieto10.

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

We have presented [*Herschel*]{} PACS observations from KINGFISH of the inner kpc of the barred spiral galaxy NGC 1097. These are the first observations to resolve a starburst ring at wavelengths probing the peak of the dust SED. We show a comparison of the ring in a variety of tracers and find similar bright knots in the mid- and far-IR and radio continuum. These knots do not correspond to the same knots traced by CO. We find modest variation azimuthally in the mid- and far-IR band ratios suggesting that either there is no azimuthal age gradient, as would be predicted by the “pearls on a string” mode of star-formation, that dust heating is dominated by an older stellar population and/or that the dust heating variations get quickly erased over the short ring orbital period ($\sim 18$ Myr). Finally, we place an order-of-magnitude tighter constraint on the far-IR emission originating in the central $\sim 600$ pc of the galaxy.

The authors thank R. Beck for the radio continuum data and P.-Y. Hsieh for the CO data. K.S. would like to thank G. van de Ven and L. Burtscher for useful discussions. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF- IFSI/OAA/OAP/OAT, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI/INAF (Italy), and CICYT/MCYT (Spain).

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[^1]: Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
