From Young Massive Clusters to Old Globular Clusters: Density Profile Evolution and IMBH Formation

TL;DR

This paper investigates how young massive clusters evolve into globular clusters and how their dense environments can lead to the formation of intermediate-mass black holes through stellar collisions and mergers.

Contribution

It demonstrates the dynamical evolution of YMCs from EFF to King profiles and explores the conditions under which runaway stellar mergers produce IMBHs, considering different mass loss scenarios.

Findings

YMCs evolve from EFF to King/Wilson profiles over time.

Runaway stellar mergers can produce IMBHs up to 4000 solar masses.

Massive star depletion accelerates core collapse, aiding IMBH formation.

Abstract

The surface brightness profiles of globular clusters are conventionally described with the well-known King profile. However, observations of young massive clusters (YMCs) in the local Universe suggest that they are better fit by simple models with flat central cores and simple power-law densities in their outer regions (such as the Elson-Fall-Freeman, or EFF, profile). Depending on their initial central density, YMCs may also facilitate large numbers of stellar collisions, potentially creating very massive stars that will directly collapse to intermediate-mass black holes (IMBHs). Using Monte Carlo $N$-body models of YMCs, we show that EFF-profile clusters transform to Wilson or King profiles through natural dynamical evolution, but that their final $W_0$ parameters do not strongly correlate to their initial concentrations. In the densest YMCs, runaway stellar mergers can produce stars that collapse into IMBHs, with their final masses depending on the treatment of the giant star envelopes during collisions. If a common-envelope prescription is assumed, where the envelope is partially or entirely lost, stars form with masses up to $824\,M_{\odot}$, collapsing into IMBHs of $232\,M_{\odot}$. Alternatively, if no mass loss is assumed, stars as massive as $4000\,M_{\odot}$ can form, collapsing into IMBHs of $\sim 4000\,M_{\odot}$. In doing so, these runaway collisions also deplete the clusters of their primordial massive stars, reducing the number of stellar-mass BHs by as much as $\sim$ 40%. This depletion will accelerate the core collapse, suggesting that the process of IMBH formation itself may produce the high densities observed in some core-collapsed clusters.