Astrophysical Implications of the Binary Black-Hole Merger GW150914 (1602.03846v1)

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.

• The paper confirms the first observed binary black-hole merger with heavy black holes (~36 and ~29 solar masses), validating key astrophysical models.
• It contrasts isolated binary evolution with dynamical interactions in dense star clusters as formation channels for binary black-hole systems.
• The analysis refines estimates on merger rates and eccentricity, offering actionable insights for future gravitational-wave observations.

Astrophysical Implications of the Binary Black-Hole Merger GW150914

The paper "Astrophysical Implications of the Binary Black-Hole Merger GW150914" provides a comprehensive analysis of the binary black-hole (BBH) merger event detected by Advanced LIGO. This landmark event offers significant insights into the formation of stellar-mass black holes (BHs), the characteristics of binary systems, and the prospects for gravitational-wave astronomy.

Key Findings and Analysis

The detection of GW150914 represents the first confirmed observation of a BBH merger. This event validates the existence of BBH systems with the capability of merging within the lifetime of the Universe, confirming several long-theorized astrophysical models. The masses of the black holes involved were notable for their magnitude, approximately 36 and 29 solar masses, which are heavier than the BHs typically observed in X-ray binaries.

The paper discusses two primary models for the formation of such BBH systems:

• Isolated Binary Evolution: This model suggests that BBHs form from massive binary star systems in galactic fields, undergoing mass transfer and common-envelope phases before collapsing into black holes.
• Dynamical Interactions in Dense Star Clusters: Alternatively, the BBHs could form through dynamical interactions in dense star clusters where massive stars rapidly segregate into the center, form binaries, and eventually merge.

Both pathways suggest that the progenitor stars of GW150914 were of low metallicity, indicating that they formed in environments with metallicity less than half of the solar value.

Implications for Astrophysics and Future Research

The implications of GW150914 are multi-faceted. From an astrophysical perspective, the confirmation of "heavy" black holes implies weak stellar winds in the progenitor's evolutionary phase, necessitating a reevaluation of stellar evolution models under varying metallicity conditions.

The estimated merger rate of such BBHs is in line with the upper predictions of existing models, guiding future expectations for the frequency of similar detections. This rate provides constraints that are crucial for refining theoretical models of black hole formation and evolution.

Prospects for Gravitational-Wave Astronomy

With Advanced LIGO's future observations, the detection capabilities will expand, potentially allowing for a greater sample size of BBH mergers. This will enhance our understanding of BH mass distributions, spin properties, and their evolution with cosmic time. Furthermore, the upper limits on eccentricity derived from the observed merger provide insights into the dynamical history of the binary systems, differentiating between potential formation channels.

The intersection of gravitational wave and electromagnetic observations could revolutionize our understanding of compact object mergers, including the possibility of identifying electromagnetic counterparts to BBH mergers. This can potentially provide more precise localizations and insights into the host environments.

Concluding Remarks

Overall, the observation of GW150914 inaugurates a new era in astrophysics, confirming decades of theoretical work and providing a fertile ground for future studies in relativistic astrophysics. The event compels advancements in both theoretical models and observational strategies, ensuring a vibrant field of research as gravitational-wave astronomy matures. The implications for cosmology, particularly regarding the formation of massive black holes and their role in galactic evolution, are profound, promising significant breakthroughs in understanding our Universe's past and future.