BPASS predictions for Binary Black-Hole Mergers

TL;DR

This study uses BPASS to predict binary black hole merger rates, mass distributions, and their dependence on metallicity, providing insights into the origins of LIGO's GW150914 event and implications for future detections.

Contribution

It offers the first detailed predictions of binary black hole merger properties as a function of metallicity using BPASS, linking stellar evolution to gravitational wave observations.

Findings

Low metallicity environments favor massive black hole mergers.

GW150914-like events are most probable in populations with Z<0.008.

At very low metallicity (Z=0.0001), a significant fraction of mergers match observed masses.

Abstract

Using the Binary Population and Spectral Synthesis code BPASS, we have calculated the rates, timescales and mass distributions for binary black hole mergers as a function of metallicity. We consider these in the context of the recently reported 1st LIGO event detection. We find that the event has a very low probability of arising from a stellar population with initial metallicity mass fraction above Z=0.010 (Z>0.5Zsun). Binary black hole merger events with the reported masses are most likely in populations below 0.008 (Z<0.4Zsun). Events of this kind can occur at all stellar population ages from ~3 Myr up to the age of the universe, but constitute only 0.1 to 0.4 per cent of binary BH mergers between metallicities of Z=0.001 to 0.008. However at metallicity Z=0.0001, 26 per cent of binary BH mergers would be expected to have the reported masses. At this metallicity the progenitor merger times can be close to ~10Gyr and rotationally-mixed stars evolving through quasi-homogeneous evolution, due to mass transfer in a binary, dominate the rate. The masses inferred for the black holes in the binary progenitor of GW\,150914 are amongst the most massive expected at anything but the lowest metallicities in our models. We discuss the implications of our analysis for the electromagnetic follow-up of future LIGO event detections.