Minimum gas mass accreted by spinning intermediate-mass black holes in stellar clusters

TL;DR

This paper estimates the minimum amount of gas an intermediate-mass black hole must accrete to achieve a high spin, providing insights into their growth mechanisms in stellar clusters.

Contribution

It introduces a method to quantify the gas mass needed for IMBHs to reach observed spins, linking accretion to black hole growth history.

Findings

IMBHs with mass >1000 M_sun and spin >0.6 require at least 100 M_sun of gas accretion.

IMBHs can only accrete up to half their mass without reaching maximal spin.

The estimates help distinguish between growth via accretion versus mergers.

Abstract

The spin of intermediate-mass black holes (IMBHs) growing through repeated black hole mergers in stellar clusters statistically asymptotes to zero. Putative observations of IMBHs with dimensionless spin parameter $\chi\gtrsim 0.6$ would require a phase of coherent gas accretion to spin up the black hole. We estimate the amount of gas necessary to produce a given IMBH spin. If the observed IMBH mass and spin are $M\gtrsim 1000~M_\odot$ and $\chi\gtrsim 0.6$, respectively, the IMBH must have coherently accreted at least $\sim 100~M_\odot$ of gas. In this scenario, as long as the spin is not maximal, the IMBH can only accrete at most half of its mass. Our estimates can constrain the relative contribution of accretion and mergers to the growth of IMBHs in dense stellar environments.