Supermassive Black Hole Formation at High Redshifts Through a Primordial Magnetic Field

TL;DR

This paper proposes that primordial magnetic fields strong enough to heat collapsing gas can prevent cooling and fragmentation in early dark matter halos, enabling rapid formation of supermassive black holes at high redshifts.

Contribution

It demonstrates that primordial magnetic fields above a certain threshold can keep gas warm during collapse, facilitating SMBH formation without fragmentation, a novel mechanism in early universe models.

Findings

Magnetic fields >3.6 nG can suppress H2 cooling in collapsing gas.

Warm gas temperature remains near 10^4 K during collapse.

Rare regions with strong magnetic fields can produce enough halos for high-redshift quasars.

Abstract

It has been proposed that primordial gas in early dark matter halos, with virial temperatures above 10^4 K, can avoid fragmentation and undergo rapid collapse, possibly resulting in a supermassive black hole (SMBH). This requires the gas to avoid cooling and to remain at temperatures near T=10^4 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 B = 3.6 (comoving) nG, the collapsing gas is kept warm (T=10^4K) until it reaches the critical density n_crit=10^3 cm^{-3} at which the roto-vibrational states of H_2 approach local thermodynamic equilibrium. H_2-cooling then remains inefficient, and the gas temperature stays near 10^4K, even as it continues to collapse to higher densities. The critical magnetic field strength required to permanently suppress H_2-cooling is somewhat higher than upper limit of approx. 2 nG from the cosmic microwave background (CMB). However, it can be realized in the rare (2-3)-sigma regions of the spatially fluctuating B-field; these regions contain a sufficient number of halos to account for the z=6 quasar BHs.