Intermediate-Mass Black Holes in Early Globular Clusters

TL;DR

This paper proposes a new formation mechanism for intermediate-mass black holes in globular clusters, suggesting they grow significantly by accreting gas during the second-generation star formation phase, influenced by cluster dynamics.

Contribution

It introduces a model where gas accretion onto seed black holes during second-generation star formation leads to IMBH growth, linking black hole development to cluster evolution.

Findings

Gas accretion can increase black hole mass by up to 100 times.

High gas density in cluster cores facilitates black hole growth.

IMBH mass may weakly depend on current cluster properties.

Abstract

Spectroscopic and photometric observations show that many globular clusters host multiple stellar populations, challenging the common paradigm that globular clusters are "simple stellar populations" composed of stars of uniform age and chemical composition. The chemical abundances of second-generation (SG) stars constrain the sources of gas out of which these stars must have formed, indicating that the gas must contain matter processed through the high-temperature CNO cycle. First-generation massive Asymptotic Giant Branch (AGB) stars have been proposed as the source of this gas. In a previous study, by means of hydrodynamical and N-body simulations, we have shown that the AGB ejecta collect in a cooling flow in the cluster core, where the gas reaches high densities, ultimately forming a centrally concentrated subsystem of SG stars. In this Letter we show that the high gas density can also lead to significant accretion onto a pre-existing seed black hole. We show that gas accretion can increase the black hole mass by up to a factor of 100. The details of the gas dynamics are important in determining the actual black hole growth. Assuming a near-universal seed black hole mass and small cluster-to-cluster variations in the duration of the SG formation phase, the outcome of our scenario is one in which the present intermediate-mass black hole (IMBH) mass may have only a weak dependence on the current cluster properties. The scenario presented provides a natural mechanism for the formation of an IMBH at the cluster center during the SG star-formation phase.