The aftermath of the first stars: massive black holes

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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 neighboring neutral minihaloes, which contain high density primordial gas that can accrete onto a Pop III remnant black hole. We find that there may have been a significant time delay, of order ~10^8 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 H_2 molecules that form in relic H II regions, combined with the continued formation of H_2 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} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1}, is established. Furthermore, we find that a high fraction of HD molecules, X_{HD} > 10^{-7}, is formed, potentially enabling the formation of Pop II.5 stars 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.

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