The influence of magnetic fields, turbulence, and UV radiation on the formation of supermassive black holes

TL;DR

This study investigates how magnetic fields, turbulence, and UV radiation influence primordial gas collapse, revealing that these factors significantly lower the UV threshold needed to prevent fragmentation and potentially facilitate supermassive black hole formation.

Contribution

It provides an analytical model showing how magnetic fields and turbulence reduce the UV radiation threshold for hot gas retention during primordial collapse, aiding SMBH seed formation.

Findings

Magnetic fields lower the critical UV intensity by a factor of ~10.

Turbulence further decreases the UV threshold, especially in massive halos.

Stronger turbulence and magnetic fields exponentially increase halos exposed to supercritical UV backgrounds.

Abstract

Context. The seeds of the supermassive black holes (SMBHs) with masses of ~10^9 M_Sun observed already at z ~ 6 may have formed through the direct collapse of primordial gas in T_vir >~ 10^4 K halos, whereby the gas must stay hot (~10^4 K) in order to avoid fragmentation. Aims. The interplay between magnetic fields, turbulence, and a UV radiation background during the gravitational collapse of primordial gas in a halo is explored; in particular, the possibilities for avoiding fragmentation are examined. Methods. Using an analytical one-zone model, the evolution of a cloud of primordial gas is followed from its initial cosmic expansion through turnaround, virialization, and collapse up to a density of 10^7 cm-3. Results. It was found that in halos with no significant turbulence, the critical UV background intensity (J_21^crit) for keeping the gas hot is lower by a factor ~10 for an initial comoving magnetic field B_0 ~ 2 nG than for the zero-field case, and even lower for stronger fields. In turbulent halos, J_21^crit is found to be a factor ~10 lower than for the zero-field-zero-turbulence case, and the stronger the turbulence (more massive halo and/or stronger turbulent heating) the lower J_21^crit. Conclusions. The reduction in J_21^crit is particularly important, since it exponentially increases the number of halos exposed to a supercritical radiation background.