The MEGaN project II. Gravitational waves from intermediate mass- and binary black holes around a supermassive black hole

TL;DR

This study models the evolution and gravitational wave signals of intermediate-mass and binary black holes near supermassive black holes, revealing their merger rates, ejection mechanisms, and observational biases in galactic nuclei.

Contribution

It provides new insights into black hole dynamics and merger rates in galactic centers using advanced N-body simulations including relativistic effects.

Findings

Stellar BHs rapidly segregate around SMBHs, forming extreme mass-ratio inspirals.

Merger rate for IMBHs orbiting SMBHs is approximately 0.03 yr$^{-1}$ Gpc$^{-3}$.

BHBs in galactic nuclei can appear up to 30% heavier due to Doppler shifts.

Abstract

We investigate the evolution of intermediate-mass (IMBHs), stellar (BHs) and binary black holes (BHBs), deposited near a supermassive black hole (SMBH) by a population of massive star clusters. Stellar BHs rapidly segregate around the SMBH, driving the formation of extreme mass-ratio inspirals that coalesce at a rate $\Gamma= 0.02-0.2$ yr$^{-1}$ Gpc$^{-3}$ at redshift $z=0$. A few IMBHs orbiting the SMBH favour the formation of massive pairs that coalescence within a Hubble time, being the merger rate for this channel $\Gamma =0.03$ yr$^{-1}$ Gpc$^{-3}$. Recoiling kicks post-merger can eject the remnant from the galaxy centre, especially in dwarf galaxies. Our results suggest that this mechanism can lead to up to $10^5$ ejected SMBH within 1 Gpc. An IMBH co-existing with a few single and binary BHs in the same cluster can affect significantly their evolution, either driving binary disruption, yielding to intermediate-mass ratio inspirals (merger rate $\Gamma =9.5$ yr$^{-1}$ Gpc$^{-3}$), or boosting BHBs coalescence ($\Gamma =2-8$ yr$^{-1}$ Gpc$^{-3}$). In a few simulations, the SMBH boosts BHBs coalescence, leading this process to a merger rate $\Gamma =1$ yr$^{-1}$ Gpc$^{-3}$. We note that BHBs experiencing a merger in a galactic nucleus can be erroneously estimated $\sim 30\%$ heavier than it really is because of the Doppler shift of the wave frequency as caused by the rapid motion around the SMBH. All our simulations are carried out using an $N$-body code tailored to treat close encounters and post-Newtonian dynamics, that includes also the galaxy field and dynamical friction in the particles' equation of motion.