Formation of supermassive black hole seeds in nuclear star clusters via gas accretion and runaway collisions

TL;DR

This study investigates how supermassive stars, potential seeds for black holes, form in early universe star clusters through gas accretion and stellar collisions, highlighting the importance of initial conditions and accretion methods.

Contribution

It demonstrates that supermassive stars of 10^3-10^5 solar masses can form via gas accretion and collisions in high-redshift star clusters under various scenarios.

Findings

Supermassive stars can form across different accretion models.

Collision timescales can rival or be shorter than accretion timescales.

Initial conditions critically influence the mass growth of supermassive stars.

Abstract

More than two hundred supermassive black holes (SMBHs) of masses $\gtrsim 10^9\,\mathrm{M_{\odot}}$ have been discovered at $z \gtrsim 6$. One promising pathway for the formation of SMBHs is through the collapse of supermassive stars (SMSs) with masses $\sim 10^{3-5}\,\mathrm{M_{\odot}}$ into seed black holes which could grow upto few times $10^9\,\mathrm{M_{\odot}}$ SMBHs observed at $z\sim 7$. In this paper, we explore how SMSs with masses $\sim 10^{3-5}\,\mathrm{M_{\odot}}$ could be formed via gas accretion and runaway stellar collisions in high-redshift, metal-poor nuclear star clusters (NSCs) using idealised N-body simulations. We explore physically motivated accretion scenarios, e.g. Bondi-Hoyle-Lyttleton accretion and Eddington accretion, as well as simplified scenarios such as constant accretions. While gas is present, the accretion timescale remains considerably shorter than the timescale for collisions with the most massive object (MMO). However, overall the timescale for collisions between any two stars in the cluster can become comparable or shorter than the accretion timescale, hence collisions still play a crucial role in determining the final mass of the SMSs. We find that the problem is highly sensitive to the initial conditions and our assumed recipe for the accretion, due to the highly chaotic nature of the problem. The key variables that determine the mass growth mechanism are the mass of the MMO and the gas reservoir that is available for the accretion. Depending on different conditions, SMSs of masses $\sim10^{3-5} \,\mathrm{M_{\odot}}$ can form for all three accretion scenarios considered in this work.