Evolution of Galactic Nuclei. I. orbital evolution of IMBH

TL;DR

This study uses large-scale N-body simulations to explore how intermediate-mass black holes (IMBHs) evolve orbitally after their parent star clusters are disrupted near galactic centers, revealing rapid mergers with supermassive black holes.

Contribution

It provides the first detailed simulation-based analysis of IMBH orbital evolution post-cluster disruption, highlighting the impact of dynamical friction and loss-cone depletion on merger timescales.

Findings

IMBHs sink toward SMBHs via dynamical friction

Dynamical friction stalls when IMBHs eject surrounding stars

Eccentricity increases, accelerating gravitational wave mergers

Abstract

Resent observations and theoretical interpretations suggest that IMBHs (intermediate-mass black hole) are formed in the centers of young and compact star clusters born close to the center of their parent galaxy. Such a star cluster would sink toward the center of the galaxy, and at the same time stars are stripped out of the cluster by the tidal field of the parent galaxy. We investigated the orbital evolution of the IMBH, after its parent cluster is completely disrupted by the tidal field of the parent galaxy, by means of large-scale N-body simulations. We constructed a model of the central region of our galaxy, with an SMBH (supermassive black hole) and Bahcall-Wolf stellar cusp, and placed an IMBH in a circular orbit of radius 0.086pc. The IMBH sinks toward the SMBH through dynamical friction, but dynamical friction becomes ineffective when the IMBH reached the radius inside which the initial stellar mass is comparable to the IMBH mass. This is because the IMBH kicks out the stars. This behavior is essentially the same as the loss-cone depletion observed in simulations of massive SMBH binaries. After the evolution through dynamical friction stalled, the eccentricity of the orbit of the IMBH goes up, resulting in the strong reduction in the merging timescale through gravitational wave radiation. Our result indicates that the IMBHs formed close to the galactic center can merge with the central SMBH in short time. The number of merging events detectable with DECIGO is estimated to be around 50 per year. Event rate for LISA would be similar or less, depending on the growth mode of IMBHs.