Formation of supermassive black holes by direct collapse in pre-galactic haloes

Abstract

We describe a mechanism by which supermassive black holes (SMBHs) can form directly in the nuclei of protogalaxies, without the need for ‘seed’ black holes left over from early star formation. Self-gravitating gas in dark matter haloes can lose angular momentum rapidly via runaway, global dynamical instabilities, the so-called ‘bars within bars’ mechanism. This leads to the rapid build-up of a dense, self-gravitating core supported by gas pressure – surrounded by a radiation pressure-dominated envelope – which gradually contracts and is compressed further by subsequent infall. We show that these conditions lead to such high temperatures in the central region that the gas cools catastrophically by thermal neutrino emission, leading to the formation and rapid growth of a central black hole. We estimate the initial mass and growth rate of the black hole for typical conditions in metal-free haloes with Tvir∼ 104-K, which are the most likely to be susceptible to runaway infall. The initial black hole should have a mass of ≲20 M⊙, but in principle could grow at a super-Eddington rate until it reaches ∼104–106 M⊙. Rapid growth may be limited by feedback from the accretion process and/or disruption of the mass supply by star formation or halo mergers. Even if super-Eddington growth stops at ∼103–104 M⊙, this process would give black holes ample time to attain quasar-size masses by a redshift of 6, and could also provide the seeds for all SMBHs seen in the present Universe.