The formation of compact massive self-gravitating discs in metal-free haloes with virial temperatures of ~ 13000-30000 K

TL;DR

This study uses hydrodynamical simulations to show that metal-free dark matter haloes with high virial temperatures can form compact, massive, self-gravitating discs, potentially leading to black hole seed formation.

Contribution

It demonstrates the formation of centrifugally supported massive discs in metal-free haloes with virial temperatures of 13000-30000 K, a process not previously detailed in this context.

Findings

Discs have scale-lengths of 0.075-0.27 pc and rotation velocities of 25-60 km/s.

Gas becomes gravitationally unstable and develops turbulence comparable to virial velocities.

Fragmentation and complex dynamical evolution occur in some haloes.

Abstract

We have used the hydrodynamical AMR code ENZO to investigate the dynamical evolution of the gas at the centre of dark matter haloes with virial velocities of ~ 20 - 30 kms and virial temperatures of ~ 13000-30000 K at z ~ 15 in a cosmological context. The virial temperature of the dark matter haloes is above the threshold where atomic cooling by hydrogen allows the gas to cool and collapse. We neglect cooling by molecular hydrogen and metals, as may be plausible if H_2 cooling is suppressed by a meta-galactic Lyman-Werner background or an internal source of Lyman-Werner photons, and metal enrichment has not progressed very far. The gas in the haloes becomes gravitationally unstable and develops turbulent velocities comparable to the virial velocities of the dark matter haloes. Within a few dynamical times it settles into a nearly isothermal density profile over many decades in radius losing most of its angular momentum in the process. About 0.1 - 1 % of the baryons, at the centre of the dark matter haloes, collapse into a self-gravitating, fat, ellipsoidal, centrifugally supported exponential disc with scale-length of ~ 0.075-0.27 pc and rotation velocities of 25-60 kms. We are able to follow the settling of the gas into centrifugal support and the dynamical evolution of the compact disc in each dark matter halo for a few dynamical times. The dynamical evolution of the gas at the centre of the haloes is complex. In one of the haloes the gas at the centre fragments into a triple system leading to strong tidal perturbations and eventually to the in-fall of a secondary smaller clump into the most massive primary clump. The formation of centrifugally supported self-gravitating massive discs is likely to be an important intermediary stage en route to the formation of a massive black hole seed.