Gas infall into atomic cooling haloes: On the formation of protogalactic discs and supermassive black holes at z > 10

Abstract

We have performed hydrodynamical simulations from cosmological initial conditions using the Adaptive Mesh Refinement (AMR) code RAMSES to study atomic cooling haloes (ACHs) at z = 10 with masses in the range 5 × 107M⊙ ⊙M ⊙ 2 × 109M⊙. We assume the gas has primordial composition and H2-cooling and prior star formation in the haloes have been suppressed. We present a comprehensive analysis of the gas and dark matter (DM) properties of 19 haloes at a spatial resolution of ~10 (proper) pc, selected from simulations with a total volume of ~2000 (comoving) Mpc3. This is the largest statistical hydro-simulation study of ACHs at z > 10 to date. We examine the morphology, angular momentum, thermodynamical state and turbulent properties of these haloes, in order to assess the prevalence of discs and massive overdensities that may lead to the formation of supermassive black holes (SMBHs). We find no correlation between either the magnitude or the direction of the angular momentum of the gas and its parent DM halo. Only three of the haloes form rotationally supported cores. Two of the most massive haloes, however, form massive, compact overdense blobs, which migrate to the outer region of the halo. These blobs have an accretion rate between ~10-1 and 10-3M⊙ yr-1 (at a distance of 100 pc from their centre), and are possible sites of SMBH formation. Our results suggest that the degree of rotational support and the fate of the gas in a halo is determined by its large-scale environment and merger history. In particular, the two haloes that form overdense blobs are located at knots of the cosmic web, cooled their gas early on (z > 17) and experienced many mergers. The gas in these haloes is thus lumpy and highly turbulent, with Mach numbers M≥ 5. In contrast, the haloes forming rotationally supported cores are relatively more isolated, located mid-way along filaments of the cosmic web, cooled their gas more recently and underwent fewer mergers. As a result, the gas in these haloes is less lumpy and less turbulent (Mach numbersM⊙ 4), and could retain most of its angular momentum. The remaining 14 haloes have a diverse range of intermediate properties. If verified in a larger sample of haloes and with additional physics to account for metals and star formation, our results will have implications for observations of the highest redshift galaxies and quasars with James Web Space Telescope. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.