Magnetic braking during direct collapse black hole formation

Magnetic fields are expected to be efficiently amplified during the formation of the first massive black holes via the small-scale dynamo and in the presence of strong accretion shocks occurring during gravitational collapse. Here, we analyze high-resolution cosmological magneto-hydrodynamical simulations of gravitational collapse in atomic cooling halos, exploring the dynamical role of magnetic fields, particularly concerning the effect of magnetic braking and angular momentum transport. We find that after the initial amplification, magnetic fields contribute to the transport of angular momentum and reduce it compared to pure hydrodynamical simulations. However, the magnetic and Reynolds torques do not fully compensate for the inward advection of angular momentum, which still accumulates over timescales of $\sim1$~Myr. A Jeans analysis further shows that magnetic pressure strongly contributes to suppressing fragmentation on scales of $0.1-10$~pc. Overall, the presence of magnetic fields thus aids in the transport of angular momentum and favors the formation of massive objects.