"Constraining the Life Cycle of Binary Supermassive Black Holes with Multi-Messenger Astrophysics"

Galaxy mergers are a fundamental foundation for much of astrophysics. Mergers are the main driver of hierarchical structure formation over cosmic time explaining the observed demographic changes in galaxies with redshift. Our understanding of the contributions of mergers to these processes, including the stellar mass growth of galaxies, is only as good as our understanding of the merger process. Given that every galaxy appears to host a central supermassive black hole (SMBH), we expect that each merger will result in the formation of a binary SMBH. While galaxy mergers are the birth place of these systems, the nanoHertz gravitational wave background, recently uncovered in pulsar timing array (PTA) data, is the end stage. Understanding the link between the cosmic binary SMBH population and the gravitational radiation detected by PTAs is crucial to our community's capability of making meaningful scientific statements using PTA observations.

In this talk, I will cover the entire life cycle of binary SMBHs and my on-going work to stitch various EM & GW observations together to inform many critical open questions in astronomy. I’ll start by covering new work updating the galaxy merger rate in the local Universe. Then I’ll explore different approaches for populating merging galaxies with co-evolving SMBHs in semi-analytic models, highlighting recent results that are in line with the emerging consensus that SMBHs may have been more massive at higher redshifts than previously thought. I’ll also explain PTA observations of the GWB and how this critical detection allows us to constrain the binary SMBH evolution timescale for the first time. Finally, I’ll end by laying out the road forward as we enter an exciting new era for low-frequency multi-messenger astrophysics.