Intermediate-mass black hole incubators. Gas accretion onto stellar black hole clusters in galactic central molecular zones

TL;DR

This paper investigates how stellar black hole clusters in galactic central molecular zones can accrete gas and potentially collapse into intermediate-mass black holes, supported by analytical estimates based on the Milky Way's conditions.

Contribution

It provides an analytical framework showing that black hole clusters in galactic centers can grow sufficiently via gas accretion to form intermediate-mass black holes.

Findings

Black hole clusters can accrete enough gas in a few million years.

Accretion can lead to collapse into intermediate-mass black holes.

Locations of observed intermediate-mass black holes align with formation scenarios.

Abstract

The stellar dynamical evolution of massive star clusters formed during starburst periods leads to the segregation of $\gtrsim10^4 M_\odot$ stellar-mass black hole sub-clusters in their centres. In gas-rich environments, such as galactic central molecular zones, these black hole clusters are likely to accrete large amounts of the gas from their surroundings, which in turn affects their internal dynamics. In this Letter we estimated the corresponding accretion rate onto the black hole cluster and its radiative feedback. We assessed whether such an accretion flow can lead to the collapse of the black hole cluster into an intermediate-mass black hole. The estimates were obtained analytically, considering the astrophysical conditions and star formation history reported for the central molecular zone of our Galaxy. We find that a stellar black hole cluster with mass $\gtrsim10^4 M_\odot$ located in the twisted ring of molecular clouds with radius $\approx100$ pc that is observed in the central molecular zone of our Galaxy can accrete about the same mass in gas on a timescale of a few million years. We suggest that this is sufficient for its subsequent collapse into an intermediate-mass black hole. Based on an estimate of the dynamical friction inspiral time, we further argue that the locations of the intermediate-mass black hole candidates recently observed in the central molecular zone are compatible with their formation therein during the last starburst period reported to have occurred $\approx1$ Gyr ago.