Connecting The Hierarchically Merging Binary Black Hole Population To Their Host Galaxies

TL;DR

This paper explores how to link hierarchical black hole mergers detected via gravitational waves to their host galaxies, especially AGN, by analyzing sky localization data and galaxy catalogs to identify their origins.

Contribution

It introduces a method to assess the fraction of hierarchical black hole mergers originating in AGN by cross-correlating gravitational wave localizations with galaxy surveys.

Findings

Clustering of GW localizations can distinguish AGN origin with ~5000 detections.

Hierarchical mergers in AGN can be identified if they dominate the observed population.

The method forecasts the ability to connect GW events to host environments in future observations.

Abstract

The detection of gravitational waves from merging black holes with masses $\sim\,80-150\,\mathrm{M_\odot}$ suggests that some proportion of black hole binary systems form hierarchically in dense astrophysical environments, as most stellar evolution models cannot explain the origin of these massive black holes through isolated binary evolution. A significant fraction of such mergers could occur in Active Galactic Nuclei disks (AGN), however connecting individual black hole mergers to host galaxies is a challenging endeavor due to large localization uncertainties. We assess the feasibility of determining the fraction of hierarchically merging black hole binaries by computing the angular cross-correlation between gravitational wave localization posteriors and galaxy catalog skymaps. We forecast when the clustering of gravitational wave sky localizations can be measured accurately enough to distinguish the AGN origin scenario from hierarchical mergers in galaxies that do not host AGN. We find that if the observed merging population is dominated by binaries formed dynamically in AGN, then this could be determined with $\mathcal{O}(5000)$ mergers detected at the sensitivity that is projected for the upcoming A\# gravitational wave detectors.