Properties of dusty tori in active galactic nuclei - II. Type 2 AGN

This paper is the second part of a work investigating the properties of dusty tori in active galactic nuclei (AGN) by means of multicomponent spectral energy distribution (SED) fitting. It focuses on low-luminosity, low-redshift (z < 0.25) AGN selected among emission line galaxies in the overlapping regions between Spitzer Wide-area Infrared Extragalactic Survey (SWIRE) and Sloan Digital Sky Survey Data Release 4 as well as X-ray, radio and mid-infrared selected type 2 AGN samples from the literature. The available multiband photometry covers the spectral range from the u band up to 160 mu m. Using a standard chi 2 minimization, the observed SED of each object is fit to a set of multicomponent models comprising a stellar component, a high optical depth (tau(9.7) >= 1.0) torus and cold emission from a starburst (SB). The torus components assigned to the majority of the objects were those of the highest optical depth of our grid of models (tau(9.7) = 10.0). The contribution of the various components (stars, torus, SB) is reflected in the position of the objects on the Infrared Array Camera (IRAC) colour diagram, with star-, torus- and SB-dominated objects occupying specific areas of the diagrams and composite objects lying in between. The comparison of type 1 (as derived from Hatziminaoglou et al.) and type 2 AGN properties is broadly consistent with the unified scheme. The estimated ratio between type 2 and 1 objects is about 2-2.5:1. The AGN accretion-to-infrared luminosity ratio is an indicator of the obscuration of the AGN since it scales down with the covering factor. We find evidence supporting the receding torus paradigm, with the estimated fraction of obscured AGN, derived from the distribution of the covering factor, decreasing with increasing optical luminosity (lambda L-5100) over four orders of magnitude. The average star formation rates (SFRs) are of similar to 10 M-circle dot yr-1 for the low-z sample, similar to 40 M-circle dot yr-1 for the other type 2 AGN and similar to 115 M-circle dot yr-1 for the quasars; this result however, might simply reflect observational biases, as the quasars under study were one to two orders of magnitude more luminous than the various type 2 AGN. For the large majority of objects with 70 and/or 160 mu m detections, an SB component was needed in order to reproduce the data points, implying that the far-infrared emission in AGN arises mostly from star formation; moreover, the SB-to-AGN luminosity ratio shows a slight trend with increasing luminosity.