Orbital Evolution of an IMBH in the Galactic Nucleus with a Massive Central Black Hole

Resent observations and theoretical interpretations suggest that IMBHs(intermediate-mass black hole) are formed in the centers of young and compactstar clusters born close to the center of their parent galaxy. Such a starcluster would sink toward the center of the galaxy, and at the same time starsare stripped out of the cluster by the tidal field of the parent galaxy. Weinvestigated the orbital evolution of the IMBH, after its parent cluster iscompletely disrupted by the tidal field of the parent galaxy, by means oflarge-scale N-body simulations. We constructed a model of the central region ofour galaxy, with an SMBH (supermassive black hole) and Bahcall-Wolf stellarcusp, and placed an IMBH in a circular orbit of radius 0.086pc. The IMBH sinkstoward the SMBH through dynamical friction, but dynamical friction becomesineffective when the IMBH reached the radius inside which the initial stellarmass is comparable to the IMBH mass. This is because the IMBH kicks out thestars. This behavior is essentially the same as the loss-cone depletionobserved in simulations of massive SMBH binaries. After the evolution throughdynamical friction stalled, the eccentricity of the orbit of the IMBH goes up,resulting in the strong reduction in the merging timescale throughgravitational wave radiation. Our result indicates that the IMBHs formed closeto the galactic center can merge with the central SMBH in short time. Thenumber of merging events detectable with DECIGO is estimated to be around 50per year. Event rate for LISA would be similar or less, depending on the growthmode of IMBHs.Comment: 12 pages, 24 figures, submitted to Ap