An unbiased survey of black hole activity in the local universe

Cosmological simulations have shown that the fraction of low-mass galaxies in today’s universe that contain a nuclear black hole is directly related to the mechanism by which massive black hole seeds formed in the early universe. We present optical emission-line measurements for an unbiased sample of local (d < 80 Mpc) galaxies in order to constrain the lower limit on the black hole occupation fraction for low-mass galaxies. Drawing from a variety of sources of optical spectroscopic data, we have obtained classifications for 310 galaxies within a 256 deg² region of sky. The host galaxies in our sample match the measured galaxy luminosity function down to M_g = -15. Using these data, we have separated the objects into four distinct activity classes: quiescent (inactive) galaxies, star-forming galaxies, LINER galaxies, and black hole-powered active galactic nuclei (AGN). A substantial fraction of the objects (~10%) have ambiguous emission-line properties, which could indicate a combination of AGN and star-forming activity. Depending on how one interprets the nature of such objects and LINERs, we find that between 2% and 10% of our sample displays evidence for massive black-hole accretion. We also have examined X-ray and near-IR source catalogs for additional evidence of black-hole accretion in our sample, which increases the lower limit a modest amount.