Gravitational Waves from Active Galactic Nuclei

Abstract: 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? I will briefly detail the many formation channels that have been proposed, before zooming in on one uniquely testable solution: the “AGN channel” In this scenario, 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).