McFACTS II: Mass Ratio--Effective Spin Relationship of Black Hole Mergers in the AGN Channel

TL;DR

This study uses the McFACTS code to explore how AGN disk and star cluster parameters influence black hole merger mass distributions, aligning with gravitational wave observations.

Contribution

It introduces a detailed simulation framework to analyze the impact of various AGN environment parameters on black hole merger characteristics.

Findings

Dense, short-lived AGN disks produce observed anti-correlation in mass ratio and effective spin.

A black hole initial mass function proportional to M^{-2} fits observations better.

A prograde-to-retrograde binary formation fraction over 90% aligns with gravitational wave data.

Abstract

We use the Monte Carlo For AGN (active galactic nucleus) Channel Testing and Simulation (McFACTS, https://www.github.com/mcfacts/mcfacts) code to study the effect of AGN disk and nuclear star cluster parameters on predicted mass distributions for LIGO-Virgo-KAGRA (LVK) compact binaries forming in AGN disks. The assumptions we vary include the black hole (BH) initial mass function, disk model, disk size, disk lifetime, and the prograde-to-retrograde fraction of newly formed black hole binaries. Broadly we find that dense, moderately short-lived AGN disks are preferred for producing a $(q,\chi_{\rm eff})$ anti-correlation like those identified from existing gravitational wave (GW) observations. Additionally, a BH initial mass function (MF $\propto M^{-2}$) is preferred over a more top-heavy MF ($M^{-1}$). The preferred fraction of prograde-to-retrograde is $>90\%$, to produce results consistent with observations.