# Picky Partners: The Pairing of Component Masses in Binary Black Hole Mergers

## TL;DR

This paper analyzes binary black hole mergers and finds that black holes tend to pair with similar masses, providing insights into their formation mechanisms based on LIGO/Virgo data.

## Contribution

It demonstrates that black hole pairings are non-random and favor similar masses, with a statistical model supporting mass ratio dependence in binary formation.

## Key findings

- Black hole pairs are more likely to have similar masses than random pairing.
- The median mass ratio of black hole binaries is approximately 0.91.
- Most detected binaries have mass ratios greater than 0.5.

## Abstract

We examine the relationship between individual black hole (BH) masses in merging binary black hole (BBH) systems. Analyzing the ten BBH detections from LIGO/Virgo's first two observing runs, we find that the masses of the component BHs comprising each binary are unlikely to be randomly drawn from the same underlying distribution. Instead, the two BHs of a given binary prefer to be of comparable mass. We show that it is $\sim 5$ times more likely that the component BHs in a given binary are always equal (to within 5\%) than that they are randomly paired. If we assume that the probability of a merger between two BHs scales with the mass ratio $q$ as $q^\beta$, so that $\beta=0$ corresponds to random pairings, we find $\beta>0$ is favored at credibility 0.987. By modeling the mass distribution, we find that the median mass ratio is $q_{50\%} = 0.91^{+0.05}_{-0.17}$ at 90\% credibility. While the pairing between BHs depends on their mass ratio, we find no evidence that it depends on the total mass of the system: it is $\sim 6$ times more likely that the pairing depends purely on the mass ratio than on the total mass. We predict that 99\% of BBHs detected by LIGO/Virgo will have mass ratios $q > 0.5$. We conclude that merging black holes do not form random pairings; instead they are selective about their partners, preferring to mate with black holes of a similar mass. The details of these selective pairings provide insight into the underlying formation channels of merging binaries.