Astrophysical Implications of the Binary Black-Hole Merger GW150914

TL;DR

The detection of GW150914 confirmed the existence of heavy binary black holes, providing insights into their formation environments, merger rates, and implications for astrophysics and future gravitational-wave observations.

Contribution

This paper reports the first observation of a binary black-hole merger, confirming theoretical predictions and constraining models of black hole formation and evolution.

Findings

Black holes with masses >25 solar masses were observed.

The merger rate is consistent with existing formation models.

The event suggests formation in low-metallicity environments.

Abstract

The discovery of the gravitational-wave source GW150914 with the Advanced LIGO detectors provides the first observational evidence for the existence of binary black-hole systems that inspiral and merge within the age of the Universe. Such black-hole mergers have been predicted in two main types of formation models, involving isolated binaries in galactic fields or dynamical interactions in young and old dense stellar environments. The measured masses robustly demonstrate that relatively "heavy" black holes ($\gtrsim 25\, M_\odot$) can form in nature. This discovery implies relatively weak massive-star winds and thus the formation of GW150914 in an environment with metallicity lower than $\sim 1/2$ of the solar value. The rate of binary black-hole mergers inferred from the observation of GW150914 is consistent with the higher end of rate predictions ($\gtrsim 1 \, \mathrm{Gpc}^{-3} \, \mathrm{yr}^{-1}$) from both types of formation models. The low measured redshift ($z \sim 0.1$) of GW150914 and the low inferred metallicity of the stellar progenitor imply either binary black-hole formation in a low-mass galaxy in the local Universe and a prompt merger, or formation at high redshift with a time delay between formation and merger of several Gyr. This discovery motivates further studies of binary-black-hole formation astrophysics. It also has implications for future detections and studies by Advanced LIGO and Advanced Virgo, and gravitational-wave detectors in space.