Galaxies and Their Black Holes: Tracing and Harnessing a Cosmic Partnership with William Brandt

Supermassive black holes (SMBHs) and galaxies grow together over cosmic time, but the physical drivers of this joint evolution remain an open question. In this talk, I will summarize a statistical, population-based approach that links long-term SMBH growth to measurable galaxy properties. The key observable we use is the sample-averaged SMBH accretion rate (BHAR), inferred from sensitive X-ray surveys and interpreted as the long-term growth of SMBH populations. We compare BHAR against well-studied galactic quantities - including stellar mass, star-formation rate, and structural compactness - using large, well-controlled samples (millions of galaxies and thousands of active galactic nuclei) and partial-correlation methods that isolate the dominant connections.

Some broad conclusions are the following: (1) across z = 0-4 the generally strongest predictor of SMBH growth is galaxy stellar mass; (2) in systems dominated by a bulge component, the SMBH accretion rate tracks star formation closely, indicating synchronized SMBH and bulge growth; and (3) among star-forming galaxies, more compact systems show enhanced BHAR, plausibly because higher central gas densities feed accretion more efficiently. I will also describe how these empirical correlations can be folded into cosmological simulations to map SMBH mass assembly - via both accretion and mergers - and to explore consequences such as the evolving SMBH mass function, SMBH-galaxy scaling relations, and the drivers of the strong decline in SMBH accretion at z < 2.

Sponsored by the McDonnell Center for the Space Sciences