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