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 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.

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