Towards the first galaxies

We find that at redshifts z [greater than or similar to] 10, HD line cooling allows strongly-shocked primordial gas to cool to the temperature of the cosmic microwave background (CMB). This temperature is the minimum value attainable via radiative cooling. Provided that the abundance of HD, normalized to the total number density, exceeds a critical level of [more or less] 10⁻⁸, the CMB temperature floor is reached in a time which is short compared to the Hubble time. We estimate the characteristic masses of stars formed out of shocked primordial gas in the wake of the first supernovae, and resulting from the virialization of dark matter haloes during hierarchical structure formation to be [more or less] 10M[solar symbol]. In addition, we show that cooling by HD enables the primordial gas in relic H II regions to cool to temperatures considerably lower than those reached via H2 cooling alone. We confirm that HD cooling is unimportant in cases where the primordial gas does not go through an ionized phase, as in the formation process of the very first stars in z [greater than or similar to] 20 minihaloes of mass [more or less] 10⁶ M[solar symbol]. As well, 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 [greater than or similar to] 20. Carrying out three-dimensional hydrodynamical simulations using GAD- GET, 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 [more or less] 10⁸ yr, between black hole formation and the onset of efficient accretion. The build-up of supermassive black holes, believed to power the z [greater than or similar to] 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.