Binary Black Holes population synthesis based on the current LVK observations

TL;DR

This paper analyzes LVK gravitational-wave data to understand the origins of binary black holes, testing models of first-generation, hierarchical, and primordial black hole populations, and finds evidence supporting multiple formation channels.

Contribution

It introduces a comprehensive population synthesis model incorporating first-generation, hierarchical, and primordial black holes, fitting the LVK data with statistical preference for multiple populations.

Findings

LVK data favors a mixed population of black holes from different origins.

Hierarchical mergers are statistically supported as a significant contributor.

Adding primordial black holes improves the model fit to observations.

Abstract

The ongoing observations from ground based gravitational-wave observatories have led to the detection of more than a hundred merger events between black holes. We use the LIGO-Virgo-KAGRA (LVK) observations from 2015 to early 2024, to test the population synthesis of these merging binaries; which will allow us to probe the formation mechanisms and environments of these black holes. We test if the current sample of binary black holes can be explained only by the merger of black holes coming from the collapse of the cores of massive stars, i.e. as just first generation black holes merging with each other. Those black holes' masses will roughly follow a power-law distribution. We also test if in addition to the merger between first generation black holes, there is evidence for a second population of black hole binaries in which at least one the binaries' members is the product of an earlier merger between black holes. These binaries are typically referred to as signals of hierarchical mergers. Such a population can possibly explain the observation of very massive black hole binaries by the LVK collaboration. We find that the LVK observations give a statistical preference in log-likelihood of up to $- 2 \Delta ln\mathcal{L} = -150$ or in log-Bayes factor of up to $ln\textrm{BF} = 71$, for the full sample of black hole binaries originating from a combination of black holes following a power-law distribution and black holes from hierarchical mergers. The ratio of black holes following a power-law mass-distribution to a mass-distribution expected from hierarchical mergers is found to be as high as one-to-one. We also consider that some of the LVK black hole merging binaries are the result of primordial black holes (PBHs), merging inside dark matter halos and in the intergalactic medium. Adding a third population is preferred. [abridged]