The aftermath of the first stars: massive black holes

We investigate the evolution of the primordial gas surrounding the first massive black holes formed by the collapse of Population III stars at redshifts z >~ 20. Carrying out three-dimensional hydrodynamical simulations using GADGET, we study the dynamical, thermal and chemical evolution of the first relic HII regions. We also carry out simulations of the mergers of relic HII regions with neighbouring neutral minihaloes, which contain high-density primordial gas that could accrete on to a Population III remnant black hole. We find that there may have been a significant time delay, of the order of ~10⁸ yr, between black hole formation and the onset of efficient accretion. The build-up of supermassive black holes, believed to power the z >~ 6 quasars observed in the Sloan Digital Sky Survey, therefore faces a crucial early bottleneck. More massive seed black holes may thus be required, such as those formed by the direct collapse of a primordial gas cloud facilitated by atomic line cooling. The high optical depth to Lyman-Werner (LW) photons that results from the high fraction of H₂ molecules that form in relic HII regions, combined with the continued formation of H₂ inside the dynamically expanding relic HII region, leads to shielding of the molecules inside these regions at least until a critical background LW flux of ~10^-24 erg s^-1 cm^-2 Hz^-1 sr^-1 is established. Furthermore, we find that a high fraction of deuterium hydride (HD) molecules, X_HD >~ 10^-7, is formed, potentially enabling the formation of Population II.5 stars, with masses of the order of ~10M_solar, during later stages of structure formation when the relic HII region gas is assembled into a sufficiently deep potential well to gravitationally confine the gas again.