The Steady State of Intermediate-Mass Black Holes Near a Supermassive Black Hole

Abstract

Aims: Investigate properties of a cluster of intermediate-mass black holes surrounding a supermassive black hole. Methods: We simulate clusters of equal-mass intermediate-mass black holes ($m_{\rm{IMBH}} = 10^{3}$ ${\rm{M_\odot}}$) initialised in a shell between $0.15\leq r$ [pc] $\leq 0.25$ centered about a supermassive black hole. We explore the influence of the cluster population and supermassive black hole mass on the merger rate, the ejection rate and the escape velocity. For $M_{\text{SMBH}} = 4\times10^{6}$ ${\rm {M}_\odot}$, we use both a Newtonian and post-Newtonian formalism, going up to the 2.5th order and including cross-terms. For the other two SMBH masses ($M_{\rm{SMBH}} = 4\times10^{5}$ ${\rm{M_\odot}}$ and $M_{\rm{SMBH}} = 4\times10^{7}$ $\rm{M_\odot}$), we model the system only taking into account relativistic effects. The simulations end once a black hole escapes the cluster, a merger occurs, or the system has evolved till $100$ Myr. Results: The post-Newtonian formalism accelerates the loss rate of intermediate-mass black holes. Ejections occur more often for lower supermassive black hole masses while more massive ones increase the rate of mergers. Although relativistic effects allow for circularisation, all merging binaries have $e \gtrsim 0.97$. Strong gravitational wave signals are suppressed during our Newtonian calculations. Weaker and more frequent signals are expected from gravitational wave radiation emitted in a fly-by. In our post-Newtonian calculations, $30/406$ of the gravitational wave events capable of being observed with LISA and $\mu$Ares are detected as gravitational wave capture binaries with the remaining being in-cluster mergers. Throughout our investigation, no IMBH-IMBH binaries were detected.