Are Most Low-Luminosity AGN Really Obscured?

TL;DR

This paper argues that many low-luminosity AGN classified as obscured are actually undetected due to dilution and radiative inefficiency, affecting their observed properties and the inferred obscured fraction.

Contribution

It introduces the idea that dilution and radiative inefficiency, not obscuration, explain many low-luminosity AGN observations and their apparent obscured fraction.

Findings

Many low-luminosity AGN are not truly obscured but are diluted or radiatively inefficient.

Selection effects cause different host galaxy types to be associated with different AGN samples.

The true obscured fraction of low-luminosity AGN may be as low as 20%.

Abstract

At low Eddington ratio (mdot), two effects make it harder to detect AGN given some selection criteria. First, even with fixed accretion physics, AGN are diluted/less luminous relative to their hosts; the magnitude of this depends on host properties and so on luminosity and redshift. Second, they may transition to a radiatively inefficient state, changing SED shape and dramatically decreasing in optical/IR luminosity. These effects lead to differences in observed AGN samples, even at fixed bolometric luminosity and after correction for obscuration. The true Eddington ratio distribution may depend strongly on luminosity, but this will be seen only in surveys robust to dilution and radiative inefficiency (X-ray or narrow-line samples); selection effects imply that AGN in optical samples will have uniformly high mdot. This also implies that different selection methods yield systems with different hosts: the clustering of faint optical/IR sources will be weaker than that of X-ray sources, and optical/IR Seyferts will reside in more disk-dominated galaxies while X-ray selected Seyferts will preferentially occupy early-type systems. If observed mdot distributions are correct, a large fraction of low-luminosity AGN currently classified as 'obscured' are in fact diluted and/or radiatively inefficient, not obscured by gas or dust. This is equally true if X-ray hardness is used as a proxy for obscuration, since radiatively inefficient SEDs near mdot~0.01 are X-ray hard. These effects can explain most of the claimed luminosity/redshift dependence in the 'obscured' AGN population, with the true obscured fraction as low as 20%.