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 μm 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 μm 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 μm and [O III] line luminosities from Seyferts and a sub-population of low ionization nuclear emission-line regions (LINERs) with significant nuclear 4.6 μm 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 μm luminosity. Our results also demonstrate that some LINERs do fit in with the expectations of the simple Unified Model.