Supermassive Star Formation in Magnetized Atomic-Cooling Gas Clouds: Enhanced Accretion, Intermittent Fragmentation, and Continuous Mergers

TL;DR

This study uses 3D magnetohydrodynamic simulations to show that magnetic fields in primordial gas clouds enhance accretion and coalescence, supporting the formation of supermassive stars and black holes in the early universe.

Contribution

It demonstrates how magnetic effects amplify accretion and fragment coalescence, promoting supermassive star formation in primordial gas clouds, a novel insight into early universe black hole origins.

Findings

Magnetic fields are quickly amplified during early accretion.

Magnetic effects increase accretion rates and fragmentation.

Fragments predominantly coalesce into the primary star.

Abstract

The origin of supermassive black holes (with $\gtrsim\!10^9\,M_{\odot}$) in the early universe (redshift $z \sim 7$) remains poorly understood. Gravitational collapse of a massive primordial gas cloud is a promising initial process, but theoretical studies have difficulty growing the black hole fast enough. We focus on the magnetic effects on star formation that occurs in an atomic-cooling gas cloud. Using a set of three-dimensional magnetohydrodynamic (MHD) simulations, we investigate the star formation process in the magnetized atomic-cooling gas cloud with different initial magnetic field strengths. Our simulations show that the primordial magnetic seed field can be quickly amplified during the early accretion phase after the first protostar formation. The strong magnetic field efficiently extracts angular momentum from accreting gas and increases the accretion rate, which results in the high fragmentation rate in the gravitationally unstable disk region. On the other hand, the coalescence rate of fragments is also enhanced by the angular momentum transfer due to the magnetic effects. Almost all the fragments coalesce to the primary star, so the mass growth rate of the massive star increases due to the magnetic effects. We conclude that the magnetic effects support the direct collapse scenario of supermassive star formation.