# Hierarchical Black Hole Mergers in Active Galactic Nuclei

## TL;DR

This paper proposes that hierarchical black hole mergers in Active Galactic Nuclei disks could explain the origins of LIGO/Virgo detected mergers, predicting specific mass and spin signatures.

## Contribution

It introduces a model where migration traps in AGN disks facilitate hierarchical black hole mergers, a novel pathway for explaining observed gravitational wave events.

## Key findings

- 40% of AGN-assisted mergers involve black holes over 50 solar masses
- Hierarchical mergers in AGNs produce characteristic spin alignments
- LIGO's GW170729 could originate from this channel

## Abstract

The origins of the stellar-mass black hole mergers discovered by LIGO/Virgo are still unknown. Here we show that, if migration traps develop in the \add{accretion} disks of Active Galactic Nuclei (AGNs) and promote the mergers of their captive black holes, the majority of black holes within disks will undergo hierarchical mergers---with one of the black holes being the remnant of a previous merger. 40% of AGN-assisted mergers detected by LIGO/Virgo will include a black hole with mass $\gtrsim 50$ M$_\odot$, the mass limit from stellar core collapse. Hierarchical mergers at traps in AGNs will exhibit black hole spins (anti-)aligned with the binary's orbital axis, a distinct property from other hierarchical channels. Our results are suggestive, although not definitive (with Odds ratio of $\sim 1$), that LIGO's heaviest merger so far, GW170729, could have originated from this channel.