The nuclear and integrated far-infrared emission of nearby Seyfert galaxies

Abstract

We present far-infrared (FIR) $70-500\,\mu$m imaging observations obtained with Herschel/PACS and SPIRE of 33 nearby (median distance of 30 Mpc) Seyfert galaxies from the Revised Shapley-Ames (RSA) catalogue. We obtain the FIR nuclear ($r=1\,$kpc and $r=2\,$kpc) and integrated spectral energy distributions (SEDs). We estimate the unresolved nuclear emission at 70 $\mu$m and we fit the nuclear and integrated FIR SEDs with a grey body model. We find that the integrated FIR emission of the RSA Seyferts in our sample is dominated by emission from the host galaxy, with dust properties similar to those of normal galaxies (non AGN). We use four criteria to select galaxies whose nuclear $70\,\mu$m emission has a significant AGN contribution: (1) elevated 70/160 $\mu$m flux ratios, (2)spatially resolved, high dust temperature gradient, (3) $70\,\mu$m excess emission with respect to the fit of the FIR SEDs with a grey body, and (4) excess of nuclear SFR obtained from $70\,\mu$m over SFR from mid-infrared indicators. 16 galaxies (48 per cent of the initial sample) satisfy at least one of these conditions, whereas 10 satisfy half or more. After careful examination of these, we select six bona fide candidates (18 per cent of the initial sample) and estimate that $\sim 40-70$ per cent of their nuclear ($r=1-2\,$kpc) $70\,\mu$m emission is contributed by dust heated by the AGN.