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We use 317,000 emission-line galaxies from the Sloan Digital Sky Survey toinvestigate line-ratio selection of active galactic nuclei (AGNs). Inparticular, we demonstrate that "star formation dilution" by HII regions causesa significant bias against AGN selection in low-mass, blue, star-forming,disk-dominated galaxies. This bias is responsible for the observed preferenceof AGNs among high-mass, green, moderately star-forming, bulge-dominated hosts.We account for the bias and simulate the intrinsic population of emission-lineAGNs using a physically-motivated Eddington ratio distribution, intrinsic AGNnarrow line region line ratios, a luminosity-dependent Lbol/L[OIII] bolometriccorrection, and the observed Mbh-sigma relation. These simulations indicatethat, in massive (log(M*/Msun) > 10) galaxies, AGN accretion is correlated withspecific star formation rate but is otherwise uniform with stellar mass. Thereis some hint of lower black hole occupation in low-mass (log(M*/Msun) < 10)hosts, although our modeling is limited by uncertainties in measuring andinterpreting the velocity dispersions of low-mass galaxies. The presence ofstar formation dilution means that AGNs contribute little to the observedstrong optical emission lines (e.g., [OIII] and Ha) in low-mass andstar-forming hosts. However the AGN population recovered by our modelingindicates that feedback by typical (low- to moderate-accretion) low-redshiftAGNs has nearly uniform efficiency at all stellar masses, star formation rates,and morphologies. Taken together, our characterization of the observationalbias and resultant AGN occupation function suggest that AGNs are unlikely to bethe dominant source of star formation quenching in galaxies, but instead arefueled by the same gas which drives star formation activity.

astrophysical journal/the astrophysical journal

The Biases of Optical Line-Ratio Selection for Active Galactic Nuclei,  and the Intrinsic Relationship between Black Hole Accretion and Galaxy Star  Formation