The aftermath of the first stars: Massive black holes

Abstract

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 H II regions. We also carry out simulations of the mergers of relic H II 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 H II regions, combined with the continued formation of H2 inside the dynamically expanding relic H II 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, XHD ≳ 10-7, is formed, potentially enabling the formation of Population II.5 stars, with masses of the order of ∼10 M⊙, during later stages of structure formation when the relic H II region gas is assembled into a sufficiently deep potential well to gravitationally confine the gas again. © 2006 RAS.