Potential Subpopulations and Assembling Tendency of the Merging Black Holes

TL;DR

This study models the black hole merger population detected by gravitational waves, revealing subpopulations with distinct properties and suggesting some originate from AGN disks, based on mass and spin distributions.

Contribution

The paper introduces phenomenological models with two subpopulations to analyze gravitational wave data, providing new constraints on black hole masses, spins, and formation environments.

Findings

Maximum stellar-origin black hole mass is about 39 solar masses.

Approximately 15% of mergers occur in dynamical environments.

Dynamical black holes tend to have higher masses and spins.

Abstract

The origins of coalescing binary black holes (BBHs) detected by the advanced LIGO/Virgo are still under debate, and clues may be present in the joint mass-spin distribution of these merger events. Here we construct phenomenological models containing two sub-populations to investigate the BBH population detected in gravitational wave observations. We find that our models can explain the GWTC-3 data rather well, and several constraints to our model are required by the data: first, the maximum mass for the component with a stellar-origin, $m_{\rm max}$, is $39.1^{+2.4}_{-2.7}M_{\odot}$ at 90\% credibility; second, about $15\%$ of the mergers happen in dynamical environments, in which $7-16\%$ of events are hierarchical mergers, and these BHs have an average spin magnitude significantly larger than the first-generation mergers, with ${\rm d}\mu_{\rm a} > 0.4 $ at $99\%$ credibility; third, the dynamical component BHs tend to pair with each other with larger total mass and higher mass ratio. An independent analysis focusing on spins is also carried out, and we find that the spin amplitude of component BHs can be divided into two groups according to a division mass $m_{\rm d} = 46.1^{+5.6}_{-5.1}M_{\odot}$. These constraints can be naturally explained by current formation channels, and our results suggest that some of the observed events were likely from AGN disks.