Bence Kocsis

Abstract:

The high rate of black hole (BH) mergers detected by LIGO/Virgo opened questions on their astrophysical origin. One possibility is the dynamical channel, in which binary formation and hardening is catalyzed by dynamical encounters in globular clusters (GCs). Previous studies have shown that the BH merger rate from the present day GC density in the Universe is lower than the observed rate. In this \textit{Letter}, we study the BH merger rate by accounting for the first time for the evolution of GCs within their host galaxies. The mass in GCs was initially $\sim 8\times$ higher, which decreased to its present value due to evaporation and tidal disruption. Many BH binaries that were ejected long before their merger, originated in GCs that no longer exist. We find that the comoving merger rate in the dynamical channel from GCs varies between $18$ to $35\,{\rm Gpc}^{-3}\,{\rm yr}^{-1}$ between redshift $z=0.5$ to $2$, and the total rate is $1$, $5$, $24$ events per day within $z=0.5$, $1$, and $2$, respectively. The cosmic evolution and disruption of GCs systematically increases the present-day merger rate by a factor $\sim 2$ relative to isolated clusters. Gravitational wave detector networks offer an unique observational probe of the initial number of GC populations and their subsequent evolution across cosmic time.

Abstract:

© 2018. The American Astronomical Society. All rights reserved.. Black hole (BH) mergers detectable with the Laser Interferometer Gravitational-wave Observatory (LIGO) can occur in active galactic nucleus (AGN) disks. Here we parameterize the merger rates, the mass spectrum, and the spin spectrum of BHs in AGN disks. The predicted merger rate spans ∼10-3-104 Gpc-1 yr-1, so upper limits from LIGO (<212 Gpc-1 yr-1) already constrain it. The predicted mass spectrum has the form of a broken power law, consisting of a pre-existing BH power-law mass spectrum and a harder power-law mass spectrum resulting from mergers. The predicted spin spectrum is multipeaked with the evolution of retrograde spin BHs in the gas disk playing a key role. We outline the large uncertainties in each of these LIGO observables for this channel and we discuss ways in which they can be constrained in the future.

Abstract:

Gravitational torques among objects orbiting a supermassive black hole drive the rapid reorientation of orbital planes in nuclear star clusters (NSCs), a process known as vector resonant relaxation. In this Letter, we determine the statistical equilibrium of systems with a distribution of masses, semimajor axes, and eccentricities. We average the interaction over the apsidal precession time and construct a Monte Carlo Markov chain method to sample the microcanonical ensemble of the NSC. We examine the case of NSCs formed by 16 episodes of star formation or globular cluster infall. We find that the massive stars and stellar mass black holes form a warped disk, while low mass stars resemble a spherical distribution with a possible net rotation. This explains the origin of the clockwise disk in the Galactic center and predicts a population of black holes (BHs) embedded within this structure. The rate of mergers among massive stars, tidal disruption events of massive stars by BHs, and BH-BH mergers are highly increased in such disks. The first two may explain the origin of the observed G1 and G2 clouds, the latter may be important for gravitational wave detections with LIGO and VIRGO. More generally, black holes are expected to settle in disks in all dense spherical stellar systems assembled by mergers of smaller systems including globular clusters.

Abstract:

Recently several gravitational wave detections have shown evidence for compact object mergers. However, the astrophysical origin of merging binaries is not well understood. Stellar binaries are typically at much larger separations than what is needed for the binaries to merge due to gravitational wave emission, which leads to the so-called final AU problem. In this letter we propose a new channel for mergers of compact object binaries which solves the final AU problem. We examine the binary evolution following gas expansion due to a weak failed supernova explosion, neutrino mass loss, core disturbance, or envelope instability. In such situations the binary is possibly hardened by ambient gas. We investigate the evolution of the binary system after a shock has propagated by performing smoothed particle hydrodynamics simulations. We find that significant binary hardening occurs when the gas mass bound to the binary exceeds that of the compact objects. This mechanism represents a new possibility for the pathway to mergers for gravitational wave events.