Title: A census of the black hole population in nearby low mass galaxies

Speaker: Prof. Elena Gallo, University of Michigan

Host: Jianfeng Wu

Time: 14:30-16:30, Thursday, Oct 17, 2019

Location: Physics Building 552

Abstract: It is commonly accepted that a supermassive black hole – as massive as several millions or even billions Suns – sits at the center of every galaxy that is as large as (or larger than) the Milky Way galaxy. Additionally, the black hole is thought to play a critical role in shaping the large-scale properties of the galaxy it lives in. It does so by blowing galaxy-scale outflows of energy and matter, which in turn affect the fate of its host galaxy gas reservoir, and, ultimately, its ability to form stars. Whether a similar feedback mechanism takes place is not obvious when it comes to dwarf galaxies, i.e. galaxies that are one tenth of a Milky Way, or smaller. Even though dwarfs make up the overwhelming majority of the galaxy population out there, it is not clear whether all dwarf galaxies host a massive black hole at their center, and, if they do, whether the properties of those black holes, and chiefly their masses, scale with the overall galaxies’ properties the same way they do in larger galaxies. Part of the uncertainty has to do with the fact that standard “dynamical techniques”, which use the motion of gas and stars around the black hole to infer its presence, rely on being able to spatially resolve the black hole's gravitational sphere of influence. Yet the fraction of dwarf galaxies that host a massive black hole is of great interest to astronomers. The reason goes beyond simple demographics; the “black hole occupation fraction” in today’s dwarf galaxies is expected to be sensitive to the very mechanism through which these giant black holes were born when the Universe was still in its infancy. Surprisingly, this remains an open question. During this talk, I will present recent results from my group addressing these questions, in an effort to build a complete census of the population of nearby, massive black holes.