The formation of massive black holes in z~30 dark matter haloes with large baryonic streaming velocities

Abstract

The origins of the ~10^9 Msun quasar supermassive black holes (BHs) at redshifts z > 6 remain a theoretical puzzle. One possibility is that they grew from ~10^5 Msun BHs formed in the 'direct collapse' of pristine, atomic-cooling (temperatures T ~ 8000 K; PAC) gas that did not fragment to form ordinary stars due to a lack of molecular hydrogen and metals. We propose that baryonic streaming---the relic relative motion of gas with respect to dark matter from cosmological recombination---provides a natural mechanism for establishing the conditions necessary for direct collapse. This effect delays the formation of the first stars by inhibiting the infall of gas into dark matter haloes; streaming velocities more than twice the root-mean-square value could forestall star formation until halo virial temperatures Tvir ~ 8000 K. The resulting PAC gas can proceed to form massive BHs by any of the mechanisms proposed in the literature to induce direct collapse in the absence of a ultraviolet background. This scenario produces haloes containing PAC gas at a characteristic redshift z ~ 30. It can explain the abundance of the most luminous quasars at z = 6, regardless of whether direct collapse occurs in nearly all or less than 1 per cent of PAC haloes.