Are Most Low-Luminosity AGN Really Obscured?
  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%.
