Abstract

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 (SF) dilution" by H II 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 luminositydependent L-bol/L [O III] bolometric correction, and the observed M-BH-sigma relation. These simulations indicate that, in massive (log(M-*/M-circle dot) greater than or similar to 10) galaxies, AGN accretion is correlated with specific star formation rate (SFR) but is otherwise uniform with stellar mass. There is some hint of lower black hole occupation in low-mass (log(M-*/M-circle dot) less than or similar to 10) hosts, although our modeling is limited by uncertainties in measuring and interpreting the velocity dispersions of low-mass galaxies. The presence of SF dilution means that AGNs contribute little to the observed strong optical emission lines (e.g.,[O III] and H alpha) 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, SFRs, 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 SF quenching in galaxies, but instead are fueled by the same gas which drives SF activity.