Counting the Unseen: Massive Black Hole Demographics from Tidal Disrution Events

University Of Utah, Salt Lake City UT

Investigators

Abstract

One of the major scientific discoveries of the last two decades is that large galaxies and their central super-massive black holes (BH) have evolved together, with each directly influencing observable properties of the other. Whether less massive galaxies have similarly coevolved with central BHs, or indeed, what fraction even possess them, is still an open question due to the difficulty in detecting intermediate mass (<100,000 solar masses) BHs. A promising approach to detect and quantify these objects utilizes the brief outbursts of light emitted during a tidal disruption event (TDE), where a BH's strong gravitational tidal forces shred a star passing too close to it. The primary aim of this project is to use existing color, stellar mass, and central stellar density measurements of low mass galaxies within 20 Mpc to model the expected frequency and luminosities of TDEs that the upcoming Vera C. Rubin Observatory are expected to detect by the thousands. These observations will reveal the fraction of low-mass galaxies that host BHs, their masses, and more broadly the primary formation mechanism of central BHs. The project will support a graduate student as well as undergraduate outreach interns. Whereas intermediate mass BHs are exceedingly difficult to detect either kinematically or through their accretion signatures, TDEs represent a robust alternative technique. However, using observed TDEs to constrain black hole “demographics” requires accurate predictions of these events as a function of galaxy properties. To first order, the parsec scale density of stars in the immediate vicinity of a BH is the key parameter for accurately predicting TDE rates, and such spatial scales can only be revolved for galaxies within ~20 Mpc. In galaxies less massive than the Milky Way the central densities on these scales (and thus TDE rates) are dominated by nuclear star clusters. This project will use nuclear density measurements combined with present-day stellar mass functions to constrain the TDE rates for a range of BH mass distributions and occupation fractions. These predictions can then be compared to data from the Zwicky Transient Factory (ZTF) and the upcoming Vera C. Rubin Observatory to determine the BH demographics in galaxies as low as 100-million solar masses for the first time. Two additional projects will also predict TDE rates in stripped nuclei surrounding massive galaxies and attempt to explain the apparent “overabundance” (roughly one of every three) of TDEs in post-starburst galaxies. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.