Supermassive black hole formation at high redshifts through a primordial magnetic field

Abstract

It has been proposed that primordial gas in early dark matter halos, with virial temperatures T vir gsim 104 K, can avoid fragmentation and undergo rapid collapse, possibly resulting in a supermassive black hole. This requires the gas to avoid cooling and to remain at temperatures near T ~ 104 K. We show that this condition can be satisfied in the presence of a sufficiently strong primordial magnetic field, which heats the collapsing gas via ambipolar diffusion. If the field has a strength above midB mid gsim3.6 (comoving) nG, the collapsing gas is kept warm (T ~ 104 K) until it reaches the critical density n crit ≈ 103 cm-3 at which the rotovibrational states of H2 approach local thermodynamic equilibrium. H2 cooling then remains inefficient and the gas temperature stays near ~104 K, even as it continues to collapse at higher densities. The critical magnetic field strength required to permanently suppress H2 cooling is somewhat higher than the upper limit of ~2 nG from the cosmic microwave background. However, it can be realized in the rare gsim(2-3)σ regions of the spatially fluctuating B field; these regions contain a sufficient number of halos to account for z ≈ 6 quasar black holes.