The Emergence of Structure in the Binary Black Hole Mass Distribution

TL;DR

This study analyzes gravitational wave data to reveal structured peaks in black hole mass distributions, supporting hierarchical merger scenarios and highlighting the potential for future insights with more observations.

Contribution

It identifies multiple peaks in black hole mass distributions and discusses their implications for hierarchical merger models, using the VAMANA mixture model framework.

Findings

Peaks at 8, 14, 26, and 45 solar masses in the chirp mass distribution.

Excess black holes at 9, 16, 30, and 57 solar masses.

Mass peaks separated by a factor of two, with a gap at 10-12 solar masses.

Abstract

We use the gravitational wave signals from binary black hole merger events observed by LIGO and Virgo to reconstruct the underlying mass and spin distributions of the population of merging black holes. We reconstruct the population using the mixture model framework VAMANA (Tiwari 2020) using observations in GWTC-2 occurring during the first two observing runs and the first half of the third run (O1, O2, and O3a). Our analysis identifies a structure in the chirp mass distribution of the observed population. Specifically, we identify peaks in the chirp mass distribution at 8, 14, 26, and 45 M and a complementary structure in the component mass distribution with an excess of black holes at masses of 9, 16, 30 and 57 M_. Intriguingly, for both the distributions, the location of subsequent peaks are separated by a factor of around two and there is a lack of mergers with chirp masses of 10-12 M. The appearance of multiple peaks is a feature of a hierarchical merger scenario when, due to a gap in the black-hole mass spectrum, a pile-up occurs at the first peak followed by mergers of lower mass black-holes to hierarchically produce higher mass black-holes. However, cross-generation merger peaks and observations with high spins are also predicted to occur in such a scenario that we are not currently observing. The results presented are limited in measurement accuracy due to small numbers of observations but if corroborated by future gravitational wave observations these features have far-reaching implications.