The Nature of Obscuration in AGN: I. Insights from Host Galaxies

We analyze a sample of 30,000 nearby obscured AGNs with optical spectra from the Sloan Digital Sky Survey and mid-IR photometry from the Wide-field Infrared Survey Explorer. Our aim is to investigate the host galaxy properties in AGNs where the nuclear activity level is indicated by the mid-IR luminosity, and to compare the results to previous studies, which have used the [OIII] emission line as the main black hole activity indicator. We first carry out a systematic study of how the mid-IR colours of AGN hosts vary as a function of age-sensitive 4000 Å break strength and optical “Eddington parameter” (L[OIII]/MBH). We find that the [3.4] [4.6] micron colour has weak dependence on Dn(4000), but strong dependence on L[OIII]/MBH. We then use a “pair-matching” technique to subtract the 4.6 micron stellar emission contributed by the host galaxy. Intrinsic 4.6 micron AGN luminosities can be recovered for most Seyferts, but only statistically for LINERs. By combining our sample of Seyferts with a sample of type 1 AGN and quasars at z < 0.7 from the SDSS, we show that the [OIII] and 4.6 micron 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 micron and [OIII] line luminosities from Seyferts as a function of a variety of host galaxy properties, finding that they are identical. If we include the total 4.6 micron flux from the LINER population, we find that more of the total [OIII] luminosity arises from bulge-dominated galaxies with old stellar populations. This supports the hypothesis that an abundant supply of gas is a pre-requisite for the formation of an obscuring “torus”.