The excess far-infrared emission of active galactic nuclei in the local Universe

We have cross-correlated the Sloan Digital Sky Survey (SDSS) second data release spectroscopic galaxy sample with the IRAS Faint Source Catalogue (FSC). Optical emission line ratios are used to classify the galaxies with reliable IRAS 60- and 100-mu m detections into active galactic nuclei (AGN) and normal star-forming galaxies. We then create subsamples of normal galaxies and AGN that are very closely matched in terms of key physical properties such as stellar mass, redshift, size, concentration and mean stellar age (as measured by absorption line indicators in the SDSS spectra). We then quantify whether there are systematic differences between the infrared (IR) luminosities of the galaxies and the AGN in the matched subsamples. We find that the AGN exhibit a significant excess in far-infrared (FIR) emission relative to the star-forming galaxies in our sample. The excesses at 60 and 100 mu m are 0.21 +/- 0.03 and 0.12 +/- 0.035 dex in log L-60/M-star and log L (100)/M-star, respectively. We then discuss whether the FIR excess is produced by radiation from the active nucleus that is absorbed by dust or, alternatively, by an extra population of young stars that is not detectable at optical wavelengths.