The Formation of Very Massive Stars in Early Galaxies and Implications for Intermediate Mass Black Holes

Abstract

We investigate the ab-initio formation of super-massive stars in a pristine atomic cooling halo. The halo is extracted from a larger self-consistent parent simulation. The halo remains metal-free and star formation is suppressed due to a combination of dynamical heating from mergers and a mild ($J_{\rm LW} \sim 2 - 10 \ J_{21}$(z)) Lyman-Werner (LW) background. We find that more than 20 very massive stars form with stellar masses greater than 1000 M$_{\odot}$. The most massive star has a stellar mass of over 6000 M$_{\odot}$. However, accretion onto all stars declines significantly after the first $\sim$ 100 kyr of evolution as the surrounding material is accreted and the turbulent nature of the gas causes the stars to move to lower density regions. We post-process the impact of ionising radiation from the stars and find that ionising radiation is not a limiting factor when considering SMS formation and growth. Rather the birth environments are highly turbulent and a steady accretion flow is not maintained within the timescale (2 Myr) of our simulations. As the massive stars end their lives as direct collapse black holes this will seed these embryonic haloes with a population of black holes with masses between approximately 300 M$_{\odot}$ and 10,000 M$_{\odot}$. Afterwards they may sink to the centre of the haloes, eventually coalescing to form larger intermediate mass black holes whose in-situ mergers will be detectable by LISA.