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

TL;DR

This study investigates the properties of obscured AGN host galaxies using mid-IR and optical data, revealing correlations between IR and emission line luminosities and supporting the unified model of AGN.

Contribution

It provides a detailed comparison of mid-IR and optical AGN indicators, demonstrating their correlation and the unbiased nature of [OIII] as an AGN activity tracer.

Findings

[3.4]-[4.6] colour depends strongly on AGN activity

[OIII] and 4.6 micron luminosities correlate over four orders of magnitude

LINERs with nuclear 4.6 micron emission fit the unified AGN model

Abstract

We analyze a sample of 30,000 nearby obscured AGNs with optical spectra from SDSS and mid-IR photometry from WISE. 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 [OIII] 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 micron contribution. By combining Seyferts with a sample of SDSS quasars at z < 0.7, we show that the [OIII] and the intrinsic AGN 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 and a sub-population of LINERs with significant nuclear 4.6 micron emission, as function of a variety of host galaxy properties, finding that they are identical. We conclude, therefore, that [OIII] as an AGN indicator shows no particular biases as compared to the 4.6 micron luminosity. Our results also demonstrate that some LINERs do fit in with the expectations of the simple Unified Model.