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

Context. The seeds of the supermassive black holes with masses of similar to 10(9) M-circle dot observed already at z similar to 6 may have formed through the direct collapse of primordial gas in T-vir greater than or similar to 10(4) K halos, whereby the gas must stay hot (similar to 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 similar to 10 for an initial comoving magnetic field B-0 similar to 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 similar to 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 super-critical radiation background.