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Register a new userMapping far-IR emission from the central kiloparsec of NGC 1097
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Using photometry of NGC 1097 from the Herschel PACS (Photodetector Array Camera and Spectrometer) instrument, we study the resolved properties of thermal dust continuum emission from a circumnuclear starburst ring with a radius ~ 900 pc. These observations are the first to resolve the structure of a circumnuclear ring at wavelengths that probe the peak (i.e. lambda ~ 100 micron) of the dust spectral energy distribution. The ring dominates the far-infrared (far-IR) emission from the galaxy - the high angular resolution of PACS allows us to isolate the rings contribution and we find it is responsible for 75, 60 and 55% of the total flux of NGC 1097 at 70, 100 and 160 micron, respectively. We compare the far-IR structure of the ring to what is seen at other wavelengths and identify a sequence of far-IR bright knots that correspond to those seen in radio and mid-IR images. The mid- and far-IR band ratios in the ring vary by less than +/- 20% azimuthally, indicating modest variation in the radiation field heating the dust on ~ 600 pc scales. We explore various explanations for the azimuthal uniformity in the far-IR colors of the ring including a lack of well-defined age gradients in the young stellar cluster population, a dominant contribution to the far-IR emission from dust heated by older (> 10 Myr) stars and/or a quick smoothing of local enhancements in dust temperature due to the short orbital period of the ring. Finally, we improve previous limits on the far-IR flux from the inner ~ 600 pc of NGC 1097 by an order of magnitude, providing a better estimate of the total bolometric emission arising from the active galactic nucleus and its associated central starburst.
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NGC 1097 is a nearby SBb galaxy with a Seyfert nucleus and a bright starburst ring. We study the physical properties of the interstellar medium (ISM) in the ring using spatially resolved far-infrared spectral maps of the circumnuclear starburst ring of NGC 1097, obtained with the PACS spectrometer on board the Herschel Space Telescope. In particular, we map the important ISM cooling and diagnostic emission lines of [OI] 63 $mu$m, [OIII] 88 $mu$m, [NII] 122 $mu$m, [CII] 158 $mu$m and [NII] 205 $mu$m. We observe that in the [OI] 63 $mu$m, [OIII] 88 $mu$m, and [NII] 122 $mu$m line maps, the emission is enhanced in clumps along the NE part of the ring. We observe evidence of rapid rotation in the circumnuclear ring, with a rotation velocity of ~220$ km s$^{-1}$ (inclination uncorrected) measured in all lines. The [OI] 63 $mu$m/[CII] 158 $mu$m ratio varies smoothly throughout the central region, and is enhanced on the northeastern part of the ring, which may indicate a stronger radiation field. This enhancement coincides with peaks in the [OI] 63 $mu$m and [OIII] 88 $mu$m maps. Variations of the [NII] 122 $mu$m/[NII] 205 $mu$m ratio correspond to a range in the ionized gas density between 150 and 400 cm$^{-3}$.
NGC 1097 is a nearby barred spiral galaxy believed to be interacting with the elliptical galaxy NGC 1097A located to its northwest. It hosts a Seyfert 1 nucleus surrounded by a circumnuclear starburst ring. Two straight dust lanes connected to the ring extend almost continuously out to the bar. The other ends of the dust lanes attach to two main spiral arms. To provide a physical understanding of its structural and kinematical properties, two-dimensional hydrodynamical simulations have been carried out. Numerical calculations reveal that many features of the gas morphology and kinematics can be reproduced provided that the gas flow is governed by a gravitational potential associated with a slowly rotating strong bar. By including the self-gravity of the gas disk in our calculation, we have found the starburst ring to be gravitationally unstable which is consistent with the observation in citet{hsieh11}. Our simulations show that the gas inflow rate is 0.17 M$_sun$ yr$^{-1}$ into the region within the starburst ring even after its formation, leading to the coexistence of both a nuclear ring and a circumnuclear disk.
We present observations of the HCN and HCO+ J=1-0 transitions in the center of the nearby spiral galaxy NGC 6946 made with the BIMA and CARMA interferometers. Using the BIMA SONG CO map, we investigate the change in the I_HCN/I_CO and I_ HCO/I_CO integrated intensity ratios as a function of radius in the central kiloparsec of the galaxy, and find that they are strongly concentrated at the center. We use the 2MASS K_S band image to find the stellar surface density, and then construct a map of the hydrostatic midplane pressure. We apply a PDR model to the observed I_HCN/I_HCO+ integrated intensity ratio to calculate the number density of molecular hydrogen in the dense gas tracer emitting region, and find that it is roughly constant at 10^5 cm^-3 across our map. We explore two hypotheses for the distribution of the dense gas. If the HCN and HCO+ emission comes from self-gravitating density peaks inside of a less dense gas distribution, there is a linear proportionality between the internal velocity dispersion of the dense gas and the size of the density peak. Alternatively, the HCN and HCO+ emission could come from dense gas homogeneously distributed throughout the center and bound by ambient pressure, similar to what is observed toward the center of the Milky Way. We find both of these hypotheses to be plausible. We fit the relationships between I_HCN, I_HCO+, and I_CO. Correlations between the hydrostatic midplane pressure and I_HCN and I_HCO+ are demonstrated, and power law fits are provided. We confirm the validity of a relation found by Blitz & Rosolowsky (2006) between pressure and the molecular to atomic gas ratio in the high hydrostatic midplane pressure regime (10^6-10^8 cm^-3 K).
A panchromatic view of the star forming ring and feeding process in the central kpc of the galaxy NGC 1097 is presented. The assembled IR to UV images at $sim$10 pc resolution allow us to characterise the population of circa 250 clusters in the ring and disentangle the network of filaments of dust and gas that enshroud and feed them. The ring is a place of intermittent star bursts over the last 100 Myr. Four major episodes covering a proto-cluster phase of eleven mid-IR sources at the molecular clouds core, and two (three) previous bursts with a time separation of 20 - 30 Myr are identified. The extinction map of the inner few kpc resolves NGC1097s two major dust lanes in bundles of narrow, $<$25 pc width, filaments running along the galaxys bar. As they approach the ring, some circularise along it, others curve to the centre to produce a nuclear spiral. We believe these are kpc-scale dust-gas streamers feeding the ring and the black-hole. The total mass in clusters formed in the ring in the last 100 Myr is $< 10^7, rm{M_odot}$, i.e. $< 1% $ of the $10^{9} M_odot$ of molecular gas in the ring; yet, at its current star formation rate, $sim1.8, rm{M_odot , yr^{-1}}$, an order of magnitude more in stellar mass should have been produced over that period. This means that the availability of gas in the ring is not the sole star formation driver, perhaps the rate at which dense gas accumulates in the ring is key.
We present an intrinsic AGN SED extending from the optical to the submm, derived with a sample of unobscured, optically luminous (vLv(5100)>10^43.5 erg/s) QSOs at z<0.18 from the Palomar Green survey. The intrinsic AGN SED was computed by removing the contribution from stars using the 11.3um polycyclic aromatic hydrocarbon (PAH) feature in the QSOs mid-IR spectra; the 1sigma uncertainty on the SED ranges between 12 and 45 per cent as a function of wavelength and is a combination of PAH flux measurement errors and the uncertainties related to the conversion between PAH luminosity and star-forming luminosity. Longwards of 20um the shape of the intrinsic AGN SED is independent of the AGN power indicating that our template should be applicable to all systems hosting luminous AGN (vLv(5100) or L_X(2-10keV) > 10^43.5 erg/s). We note that for our sample of luminous QSOs, the average AGN emission is at least as high as, and mostly higher than, the total stellar powered emission at all wavelengths from the optical to the submm. This implies that in many galaxies hosting powerful AGN, there is no `safe broadband photometric observation (at lambda<1000um) which can be used in calculating star-formation rates without subtracting the AGN contribution. Roughly, the AGN contribution may be ignored only if the intrinsic AGN luminosity at 5100 Ang is at least a factor of 4 smaller than the total infrared luminosity (L_IR; 8-1000um) of the galaxy. Finally, we examine the implication of our work in statistical studies of star-formation in AGN host galaxies.
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