Physics Colloquium – Nicholas Stone

Gravitational Waves from Galactic Nuclei

Nicholas Stone, Assistant Professor
Department of Astronomy, University of Wisconsin-Madison

The discovery of GW150914 inaugurated the era of gravitational wave (GW) astronomy, opening a new window to study our Universe’s compact objects and through which to test general relativity. Now, a decade later, the LIGO-Virgo-KAGRA (LVK) collaboration has seen hundreds of GW signals, overwhelmingly from mergers of binary stellar mass black holes. Despite the many successes of GW astronomy, a zeroth-order astrophysical question remains unanswered: what astrophysical environments produce the LVK binary black holes, and by what process are they assembled? Although many formation channels have been proposed, one uniquely testable solution is the “AGN channel:” a scenario in which individual black holes pair up and merge in the dissipative gaseous environment of an active galactic nucleus. I will review recent progress in understanding the ecology of stars and black holes embedded in the massive gas disks of AGN, an astrophysics problem not unlike that describing planet formation in protoplanetary gas disks. I will then describe the unique predictions of the AGN channel for the LVK population of binary black hole mergers, focusing both on GW signatures as well as electromagnetic counterparts, both direct (transient) and indirect (statistical). Finally, I will briefly discuss the implications of the AGN channel for a future class of GW sources: the extreme mass ratio inspirals visible to future mHz space-based interferometers such as LISA, which are produced when stellar mass black holes inspiral into the supermassive ones at the very centers of active or quiescent galactic nuclei.