2008ApJ...688..794S

We have selected a sample of X-ray-emitting active galactic nuclei (AGNs) in low-mass host galaxies (∼5x10⁹-2x10¹⁰ M_☉) out to z∼1. By comparing to AGNs in more massive hosts, we have found that the AGN spatial number density and the fraction of galaxies hosting AGNs depends strongly on the host mass, with the AGN host mass function peaking at intermediate mass and with the AGN fraction increasing with host mass. AGNs in low-mass hosts show strong cosmic evolution in comoving number density, the fraction of such galaxies hosting active nuclei, and the comoving X-ray energy density. The integrated X-ray luminosity function is used to estimate the amount of the accreted black hole mass in these AGNs and places a strong lower limit of 12% to the fraction of local low-mass galaxies hosting black holes, although a more likely value is probably much higher (>50%) once the heavily obscured objects missed in current X-ray surveys are accounted for.