Black Hole Binaries from Globular Clusters and Galactic Nuclei

by Prof. Hyung Mok Lee (Dept. of Physics and Astronomy 
Seoul National University

Aula Conversi (Dipartimento di Fisica - Ed. G.Marconi)

Aula Conversi

Dipartimento di Fisica - Ed. G.Marconi

Performing N-body simulations, we examine the dynamics of BH-BH (10 msun~each) and NS-NS (1.4 msun~each) binaries formed in a cluster and its implications for gravitational wave detection. A significant fraction of compact binaries are ejected from a globular cluster after core collapse. Among the total number of ejected compact objects, 30 per cent of them are in binaries. Merging time-scales of ejected binaries, which depend on the cluster's velocity dispersion, are in some cases shorter than the age of the universe. During the merging event, these dynamically formed compact mergers are expected to produce gravitational waves that can be detectable by the advanced ground-based interferometers. Based on our reference assumptions, merger rates of ejected BH-BH and NS-NS binaries per globular cluster are estimated to be 2.5 and 0.27 per Gyr, respectively. Assuming the spatial density of globular clusters to be 8.4$h^3$ clusters Mpc$^{-3}$ and extrapolating the merger rate estimates to the horizon distance of the advanced LIGO-Virgo network, we expect the detection rates for BH-BH and NS-NS binaries with cluster origin are to be 15 and 0.024 yr$^{-1}$, respectively. We find out that some of the dynamically formed binaries are ejected with a large escape velocity. They can be responsible for short gamma-ray bursts whose locations are far from host galaxies. If time permits, I also would like to present our recent work on the eccentric binaries formed in the galactic nuclei star cluster surrounded by supermassive black holes. Binaries with extreme eccentricity and very short pericenter can be formed in dense stellar cluster in galactic nuclei and they can keep substantial eccentricity until the last moment of merger, but frequency of such binaries expected in advanced LIGO-Virgo would be much smaller than previously suggested.