Magnetic fields during the formation of supermassive black holes

Abstract

Observations of quasars at z > 6 report the existence of a billion solar mass black holes. Comprehending their formation in such a short time-scale is a matter of ongoing research. One of the most promising scenarios to assemble supermassive black holes is a monolithic collapse of protogalactic gas clouds in atomic cooling haloes with Tvir ≥ 104 K. In this paper, we study the amplification and impact of magnetic fields during the formation of seed black holes in massive primordial haloes. We perform high-resolution cosmological magnetohydrodynamic simulations for four distinct haloes and follow their collapse for a few free-fall times until the simulations reach a peak density of 7 × 10-10 g cm-3. Our findings show that irrespective of the initial seed field, the magnetic field strength reaches a saturated state in the presence of strong accretion shocks. Under such conditions, the growth time becomes very short and amplification occurs rapidly within a small fraction of the free-fall time. We find that the presence of such strong magnetic fields provides additional support against gravity and helps in suppressing fragmentation. Massive clumps of a few hundred solar masses are formed at the end of our simulations and high accretion rates of 1M⊙ yr-1 are observed. We expect that in the presence of such accretion rates, the clumps will grow to form supermassive stars of ~105M⊙. Overall, the role of the magnetic fields seems supportive for the formation of massive black holes. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.