Massive black hole formation in dense stellar environments: Enhanced X-ray detection rates in high velocity dispersion nuclear star clusters

TL;DR

This study uses Chandra X-ray data to investigate how dense nuclear star clusters with high velocity dispersions are more likely to host massive black holes, supporting theories of in-situ black hole formation.

Contribution

It provides observational evidence linking high velocity dispersion in nuclear star clusters to increased black hole formation rates, validating theoretical models.

Findings

NSCs above 40 km/s velocity dispersion are twice as likely to be X-ray detected.

X-ray detection rates are higher in dense, high-velocity NSCs, supporting black hole formation theories.

Results suggest a local formation pathway for massive black holes in dense stellar environments.

Abstract

We analyze Chandra X-ray Observatory imaging of 108 galaxies hosting nuclear star clusters (NSCs) to search for signatures of massive black holes (BHs). NSCs are extremely dense stellar environments with conditions that can theoretically facilitate massive BH formation. Recent work by Stone et al. (2017) finds that sufficiently dense NSCs should be unstable to the runaway growth of a stellar mass BH into a massive BH via tidal captures. Furthermore, there is a velocity dispersion threshold ($40\;\rm{km\;s^{-1}}$) above which NSCs should inevitably form a massive BH. To provide an observational test of these theories, we measure X-ray emission from NSCs and compare to the measured velocity dispersion and tidal capture runaway timescale. We find that NSCs above the $40\;\rm{km\;s^{-1}}$ threshold are X-ray detected at roughly twice the rate of those below (after accounting for contamination from X-ray binaries). These results are consistent with a scenario in which dense, high-velocity NSCs can form massive BHs, providing a formation pathway that does not rely on conditions found only at high redshift.