The impact of ionizing radiation on the formation of a supermassive star in the early Universe

TL;DR

This study uses radiation hydrodynamical simulations to show that ionizing radiation from nearby galaxies generally does not hinder the formation of supermassive stars in the early Universe, due to shielding effects and increased gas density.

Contribution

It provides new insights into how ionizing radiation influences supermassive star formation, highlighting shielding and density effects that promote collapse despite radiation.

Findings

Shielding by structures around the galaxy protects gas clouds.

Ionizing radiation can accelerate cloud collapse under certain conditions.

Most cases show ionizing radiation does not prevent supermassive star formation.

Abstract

A massive primordial halo near an intensely star-forming galaxy may collapse into a supermassive star (SMS) and leave a massive black hole seed of about $10^5~M_{\odot}$. To investigate the impact of ionizing radiation on the formation of an SMS from a nearby galaxy, we perform three-dimensional radiation hydrodynamical simulations by selecting a pair of massive dark matter halos forming at $z >10$. We find that rich structures such as clumps and filaments around the source galaxy shield the cloud from ionizing radiation. In fact, in some cases, cloud collapse is accelerated under ionizing radiation. This fact suggests that the ionization of the cloud's surroundings helps its collapse. Only strong radiation at the early stage of structure formation can halt the cloud collapse, but this is much stronger than observationally allowed value. We also explored the effect of ionizing radiation on a sample of 68 halos by employing an analytical model and found that increase in the mean density of the gas between the SMS forming cloud and the source galaxy protects the gas cloud from ionizing radiation as they approach each other. Thus, we conclude that ionizing radiation does not prevent the formation of an SMS in most of the cases.