Merging black hole binaries in galactic nuclei: implications for advanced-LIGO detections (1606.04889v2)

Abstract: Motivated by the recent detection of gravitational waves from the black hole binary merger GW150914, we study the dynamical evolution of black holes in galactic nuclei where massive star clusters reside. With masses of ~10^7M_Sun and sizes of only a few parsecs, nuclear star clusters are the densest stellar systems observed in the local universe and represent a robust environment where (stellar mass) black hole binaries can dynamically form, harden and merge. We show that due to their large escape speeds, nuclear star clusters can keep a large fraction of their merger remnants while also evolving rapidly enough that the holes can sink back to the central regions where they can swap in new binaries that can subsequently harden and merge. This process can repeat several times and produce black hole mergers of several tens of solar masses similar to GW150914 and up to a few hundreds of solar masses, without the need of invoking extremely low metallicity environments or implausible initial conditions. We use a semi-analytical approach to describe the formation and dynamics of black holes in massive star clusters. We find a black hole binary merger rate per volume from nuclear star clusters of ~1.5 Gpc^-3 yr^-1, implying up to a few tens of possible detections per year with Advanced LIGO. Our models suggest a local merger rate of 0.3- 1 Gpc^-3 yr^-1 for high mass black hole binaries similar to GW150914 (total mass >~ 50 M_Sun, redshift z< 0.3); a merger rate comparable to that of high mass black hole binaries that are dynamically assembled in globular clusters. Finally, we show that if all black holes receive high natal kicks, >~50km s^-1, then nuclear star clusters could dominate the local merger rate of binary black holes compared to the merger rate of similar binaries produced in either globular clusters or through isolated binary evolution.

Merging Black Hole Binaries in Galactic Nuclei: Implications for Advanced-LIGO Detections

The paper by Antonini and Rasio explores the dynamic evolution and merger rates of stellar-mass black holes (BHs) within nuclear star clusters (NSCs) and their prospects for detection by advanced-LIGO (aLIGO). Motivated by the groundbreaking observation of gravitational waves from the GW150914 event, the authors scrutinize the potential of NSCs as host environments for merging black hole binaries (BHBs), drawing comparisons with globular clusters (GCs).

The intrinsic appeal of NSCs lies in their dense, high-velocity environments, making them conducive to the retention and successive mergers of BHBs. NSCs, dense clusters within the nuclei of galaxies, are characterized by significant masses (~10^7 M_☉) and high central escape velocities. These conditions favor the formation, hardening, and merging of BHBs, significantly divergent from lower-mass GCs with lower escape velocities.

Methodology and Model Insights

The authors employ a semi-analytical approach to model the dynamical processes within NSCs, incorporating BH natal kick velocities, binary formation via both three-body interactions and hard stellar encounters, and recoil dynamics. Notably, Antonini and Rasio argue that NSCs can retain a sizable fraction of merger remnants due to their substantial escape speeds, contrasting the dynamics in GCs where mergers are often ejected.

The paper postulates that NSCs could yield a significant rate of BHB mergers — potentially comparable in mass to GW150914 — without necessitating scenarios of extremely low metallicity. The model predicts a BHB merger rate from NSCs of approximately 1.5 Gpc^-3 yr^-1, suggesting a compelling number of potentially observable events per year with aLIGO, even with conservative assumptions about natal kicks and binary formation rates.

Results and Implications

The research indicates that NSCs not only contribute substantially to local BHB mergers but also serve as a critical site for high-mass BHB mergers, where the masses can range from tens to hundreds of solar masses. Importantly, if BHs gain substantial natal kicks (~50 km/s), NSCs could dominate the local merger rate compared to GCs.

Antonini and Rasio's paper highlights the relevance of NSCs in gravitational wave astronomy, providing a framework to reconcile observational data with theoretical predictions. This work enhances our understanding of BHB origins, suggesting that NSCs, potentially more so than GCs, catalyze dynamically formed BHB mergers observable by aLIGO.

Future Directions

This research lays the groundwork for further exploration around the role of central massive black holes (MBHs) in NSCs and their interaction with BHB populations. Such inquiries could refine merger rate estimates further, leveraging high-resolution simulations. Also, continuous or episodic star formation in NSCs warrants deeper paper to assess its impact on BHB formation and merger statistics.

Overall, Antonini and Rasio provide a detailed account of the dynamics that drive BHB mergers in NSCs, positioning these stellar environments as pivotal to enhancing our comprehension of advanced gravitational wave detections.