Mass-gap Mergers in Active Galactic Nuclei

TL;DR

This paper investigates how hierarchical black hole mergers in active galactic nucleus disks can explain recent gravitational wave events with unusual masses and spins, using simulations and semi-analytical models.

Contribution

It provides a detailed comparison of observed gravitational wave events with theoretical models of mergers in AGN disks, highlighting the role of hierarchical mergers and accretion.

Findings

High-mass GW190521 consistent with high-generation BH mergers in AGN disks.

Spin properties of GW190521 align with AGN disk merger predictions.

Lower mass gap events like GW190814 can be explained by mergers involving accretion in AGN disks.

Abstract

The recently discovered gravitational wave sources GW190521 and GW190814 have shown evidence of BH mergers with masses and spins that could be outside of the range expected from isolated stellar evolution. These merging objects could have undergone previous mergers. Such hierarchical mergers are predicted to be frequent in active galactic nuclei (AGN) disks, where binaries form and evolve efficiently by dynamical interactions and gaseous dissipation. Here we compare the properties of these observed events to the theoretical models of mergers in AGN disks, which are obtained by performing one-dimensional $N$-body simulations combined with semi-analytical prescriptions. The high BH masses in GW190521 are consistent with mergers of high-generation (high-g) BHs where the initial progenitor stars had high metallicity, 2g BHs if the original progenitors were metal-poor, or 1g BHs that had gained mass via super-Eddington accretion. Other measured properties related to spin parameters in GW190521 are also consistent with mergers in AGN disks. Furthermore, mergers in the lower mass gap or those with low mass ratio as found in GW190814 and GW190412 are also reproduced by mergers of 2g-1g or 1g-1g objects with significant accretion in AGN disks. Finally, due to gas accretion, the massive neutron star merger reported in GW190425 can be produced in an AGN disk.