At low Eddington ratios ( \dot { m } ) , two effects make it more difficult to detect certain AGN given a particular set of selection methods .
First , even allowing for fixed accretion physics , at low \dot { m } AGN become less luminous relative to their hosts , diluting their emission ; the magnitude of the dilution depends on host properties and , therefore , on luminosity and redshift .
Second , low- \dot { m } systems are expected and observed to transition to a radiatively inefficient state , which changes the SED shape and dramatically decreases the luminosity at optical through infrared wavelengths .
The effects of dilution are unavoidable , while the precise changes in accretion physics at low \dot { m } are somewhat uncertain , but potentially very important for our understanding of AGN .
These effects will have different implications for samples with different selection criteria , and generically lead to differences in the AGN populations recovered in observed samples , even at fixed bolometric luminosity and after correction for obscuration .
Although the true Eddington ratio distribution may depend strongly on mass/luminosity , this will be seen only in surveys robust to dilution and radiative inefficiency , such as X-ray or narrow-line samples ; by contrast , selection effects imply that AGN in optical samples will have uniformly high Eddington ratios , with little dependence on luminosity , even at low L _ { bol } where the median ‘ ‘ true ’ ’ \dot { m } \lesssim 0.01 .
These same selection effects also imply that different selection criteria pick out systems with different hosts : as a result , the clustering of low-luminosity optical/infrared 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 be preferentially in early-type systems .
Taken together , these effects can naturally explain long-standing , apparently contradictory claims in the literature regarding AGN Eddington ratio distributions , host populations , and clustering .
Finally , we show that if current observed Eddington ratio distributions are correct , a large fraction of low-luminosity AGN currently classified as ‘ ‘ obscured ’ ’ are in fact radiatively 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 \dot { m } \sim 0.01 are characteristically 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 \sim 20 \% .