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

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.