The Biases of Optical Line-Ratio Selection for Active Galactic Nuclei,
  and the Intrinsic Relationship between Black Hole Accretion and Galaxy Star
  Formation
  We use 317,000 emission-line galaxies from the Sloan Digital Sky Survey to
investigate line-ratio selection of active galactic nuclei (AGNs). In
particular, we demonstrate that "star formation dilution" by HII regions causes
a significant bias against AGN selection in low-mass, blue, star-forming,
disk-dominated galaxies. This bias is responsible for the observed preference
of AGNs among high-mass, green, moderately star-forming, bulge-dominated hosts.
We account for the bias and simulate the intrinsic population of emission-line
AGNs using a physically-motivated Eddington ratio distribution, intrinsic AGN
narrow line region line ratios, a luminosity-dependent Lbol/L[OIII] bolometric
correction, and the observed Mbh-sigma relation. These simulations indicate
that, in massive (log(M*/Msun) > 10) galaxies, AGN accretion is correlated with
specific star formation rate but is otherwise uniform with stellar mass. There
is some hint of lower black hole occupation in low-mass (log(M*/Msun) < 10)
hosts, although our modeling is limited by uncertainties in measuring and
interpreting the velocity dispersions of low-mass galaxies. The presence of
star formation dilution means that AGNs contribute little to the observed
strong optical emission lines (e.g., [OIII] and Ha) in low-mass and
star-forming hosts. However the AGN population recovered by our modeling
indicates that feedback by typical (low- to moderate-accretion) low-redshift
AGNs has nearly uniform efficiency at all stellar masses, star formation rates,
and morphologies. Taken together, our characterization of the observational
bias and resultant AGN occupation function suggest that AGNs are unlikely to be
the dominant source of star formation quenching in galaxies, but instead are
fueled by the same gas which drives star formation activity.
