The nature of obscuration in AGN - I. Insights from host galaxies

We analyse a sample of 30 000 nearby obscured active galactic nuclei (AGNs) with optical spectra from Sloan Digital Sky Survey (SDSS) and mid-IR photometry from Wide-field Infrared Survey Explorer. Our aim is to investigate the AGN host galaxy properties with mid-IR luminosities as AGN activity indicator, and to compare with previous studies based on [O iii] emission lines. First we find that the [3.4]-[4.6] colour has weak dependence on host stellar age, but strong dependence on AGN activity. We then use a 'pair-matching' technique to subtract the host 4.6 mu m contribution. By combining Seyferts with a sample of SDSS quasars at z < 0.7, we show that the [O iii] and the intrinsic AGN 4.6 mu m luminosities correlate roughly linearly over 4 orders of magnitude, but with substantial scatter. We also compare the partition functions of the total integrated 4.6 mu m and [O iii] line luminosities from Seyferts and a sub-population of low ionization nuclear emission-line regions (LINERs) with significant nuclear 4.6 mu m emission, as a function of a variety of host galaxy properties, finding that they are identical. We conclude, therefore, that [O iii] as an AGN indicator shows no particular biases as compared to the 4.6 mu m luminosity. Our results also demonstrate that some LINERs do fit in with the expectations of the simple Unified Model.