“The Local Black Hole Occupation Fraction”

Elena Gallo, University of Michigan Department of Astronomy

Whereas mergers and intermittently efficient accretion in larger black holes spur growth and remove observational signatures of their birth, smaller black holes have more subdued histories and undergo mostly secular evolution. Consequently, both the mass distribution and the very rate of occupance of massive black holes in lower-mass galaxies contain archaeological information on the initial “seeding” mechanism.

I will present recent results from a number of observational campaigns aimed at characterizing the AGN population in nearby low-mass galaxies down to the lowest measurable accretion-powered luminosities. Most notably, I will discuss how, making use of a novel Bayesian methodology, a measurement of the local AGN fraction can be employed to yield the first, quantitative constraints on the massive black hole occupation fraction (i.e., regardless of activity level). As this measurement relies on Chandra X-ray Telescope data for hundreds of nearby galaxies, it enables us to concurrently quantify the incidence and intensity of low-level black hole accretion as a function of host stellar mass as well as environment (such as living in isolation vs. belonging to a cluster or group), and characterize the relation between massive black holes and nuclear star clusters, also as a function of environment and galactic morphology.