  We have selected a sample of X-ray emitting active galactic nuclei (AGNs) in
low-mass host galaxies (5e9-2e10 Msun) 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, though a more likely value is probably much
higher (> 50%) once the heavily-obscured objects missed in current X-ray
surveys are accounted for.
