DIRECT FORMATION OF SUPERMASSIVE BLACK HOLES IN METAL-ENRICHED GAS AT THE HEART OF HIGH-REDSHIFT GALAXY MERGERS

Abstract

We present novel 3D multi-scale smoothed particle hydrodynamics (SPH) simulations of gas-rich galaxy mergers between the most massive galaxies at z ∼ 8-10, designed to scrutinize the direct collapse formation scenario for massive black hole seeds proposed in Mayer et al. The simulations achieve a resolution of 0.1 pc, and include both metallicity-dependent optically thin cooling and a model for thermal balance at high optical depth. We consider different formulations of the SPH hydrodynamical equations, including thermal and metal diffusion. When the two merging galaxy cores collide, gas infall produces a compact, optically thick nuclear disk with densities exceeding 10-10 g cm3. The disk rapidly accretes higher angular momentum gas from its surroundings reaching ∼5 pc and a mass of ≳109 Mȯ in only a few 104 years. Outside ≳2 pc it fragments into massive clumps. Instead, supersonic turbulence prevents fragmentation in the inner parsec region, which remains warm (∼3000-6000 K) and develops strong non-axisymmetric modes that cause prominent radial gas inflows (>104 Mȯ yr-1), forming an ultra-dense massive disky core. Angular momentum transport by non-axisymmetric modes should continue below our spatial resolution limit, quickly turning the disky core into a supermassive protostar which can collapse directly into a massive black hole of mass 108-109 Mȯ via the relativistic radial instability. Such a "cold direct collapse" explains naturally the early emergence of high-z QSOs. Its telltale signature would be a burst of gravitational waves in the frequency range of 10-4-10-1 Hz, possibly detectable by the planned eLISA interferometer.