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Astrophysical Implications of the Binary Black-Hole Merger GW150914

The LIGO Scientific Collaboration, the Virgo Collaboration

TL;DR

GW150914 demonstrates the first direct binary black-hole merger and reveals heavy stellar-mass BHs formed in low-metallicity environments, consistent with weaker winds enhancing remnant masses. The work situates this event within two principal formation channels—isolated binaries and dynamical formation in dense clusters—and interprets the masses, spins, and distance to assess their viability under plausible parameter choices. It derives a local BBH merger-rate density of $2$–$400\,\mathrm{Gpc}^{-3}\,\mathrm{yr}^{-1}$, broadly compatible with theoretical predictions, and discusses how additional detections will tighten constraints on winds, natal kicks, and common-envelope evolution, with implications for the stochastic GW background and future space-based GW missions. The study thus establishes a foundational framework for gravitational-wave astrophysics and highlights the key physics needed to interpret a growing BBH merger catalog.

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.

Astrophysical Implications of the Binary Black-Hole Merger GW150914

TL;DR

GW150914 demonstrates the first direct binary black-hole merger and reveals heavy stellar-mass BHs formed in low-metallicity environments, consistent with weaker winds enhancing remnant masses. The work situates this event within two principal formation channels—isolated binaries and dynamical formation in dense clusters—and interprets the masses, spins, and distance to assess their viability under plausible parameter choices. It derives a local BBH merger-rate density of –, broadly compatible with theoretical predictions, and discusses how additional detections will tighten constraints on winds, natal kicks, and common-envelope evolution, with implications for the stochastic GW background and future space-based GW missions. The study thus establishes a foundational framework for gravitational-wave astrophysics and highlights the key physics needed to interpret a growing BBH merger catalog.

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 () 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 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 () from both types of formation models. The low measured redshift () 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.

Paper Structure

This paper contains 12 sections, 1 equation, 4 figures.

Table of Contents

  1. Introduction
  2. The properties of GW150914
  3. Black-Hole Masses in Merging Binaries
  4. Brief Review of Measured BH Masses
  5. Predicted Masses for Single BHs
  6. BBH Masses from Isolated Binary Systems
  7. BBH Masses from Dense Stellar Environments
  8. Binary Black-Hole Mergers in the Nearby Universe
  9. Binary Eccentricity and Black-Hole Spins
  10. Binary Black-Hole Merger Rates
  11. The Path Forward for Future Studies
  12. Conclusions

Figures (4)

  • Figure 1: Left: dependence of maximum BH mass on metallicity $Z$, with $Z_\odot = 0.02$ for the old (strong) and new (weak) massive star winds 2010ApJ...714.1217B. Right: compact-remnant mass as a function of zero-age main-sequence (ZAMS; i.e., initial) progenitor mass for a set of different (absolute) metallicity values 2015MNRAS.451.4086S. The masses of GW150914 are indicated by the horizontal bands.
  • Figure 2: Predictions of BBH merger rate in the comoving frame ($\mathrm{Gpc}^{-3}\,\mathrm{yr}^{-1}$) from isolated binary evolution as a function of redshift for different metallicity values (adopted from Figure 4 in 2013ApJ...779...72D). At a given redshift, the total merger rate is the sum over metallicity. The redshift range of GW150914 is indicated by the vertical band; the range of the BBH rate estimates and the redshift out to which a system like GW150914 could have been detected in this observing period are indicated by an open blue rectangular box.
  • Figure 3: Allowed initial BBH semimajor axis and eccentricity in order to merge within 10 Gyr (left of the thick solid blue line) for a BBH with the GW150914 masses. The thin solid lines with circles represent the evolutionary trajectories of individual example systems, starting at the edge of the allowed range (the circles give the time to merger of $\log t$/yr = 1, 2, 3, 4 ... 10, from left to right). The dashed lines denote periastron separations of 10, 20, and 40 ${{\rm R}_\odot}$ (left to right: orange, yellow, purple). The green dotted line shows the trajectory of a binary that has a remaining eccentricity of 0.1 at a GW frequency of 10 Hz.
  • Figure 4: Left: Horizon distance (left axis) and horizon redshift (right axis) as a function of total mass (bottom axis) and chirp mass (top axis), for equal mass, non-spinning BBH mergers. The (expected) increase in detector sensitivity with time is shown by the different lines and the chirp mass of GW150914 is indicated with a red star. Right: the same, but now for detection-weighted sensitive comoving volume, defined to yield the expected number of detections if multiplied with a merger rate per unit volume. For details see Appendix.