REIONIZATION IN A COLD DARK-MATTER UNIVERSE - THE FEEDBACK OF GALAXY FORMATION ON THE INTERGALACTIC MEDIUM

We study the coupled evolution of the intergalactic medium (IGM) and t he emerging structure in the universe in the context of the cold dark matter (CDM) model, with a special focus on the consequences of imposi ng reionization and the Gunn-Peterson constraint as a boundary conditi on on the model. We have calculated the time-varying density of the IG M by coupling our detailed, numerical calculations of the thermal and ionization balance and radiative transfer in a uniform, spatially aver aged IGM of H and He, including the mean opacity of an evolving distri bution of gas clumps which correspond to quasar absorption line clouds , to the linearized equations for the growth of density fluctuations i n both the gaseous and dark matter components in a CDM universe. We us e the linear growth equations to identify the fraction of the gas whic h must have collapsed out at each epoch, an approach similar in spirit to the so-called Press-Schechter formalism. We identify the IGM densi ty with the uncollapsed baryon fraction. The collapsed fraction is pos tulated to be a source of energy injection into the IGM, by radiation or bulk hydrodynamical heating (e.g., via shocks) or both, at a rate w hich is marginally enough to satisfy the Gunn-Peterson constraint at z < 5. Our results include the following: (1) We find that the IGM in a CDM model must have contained a substantial fraction of the total bar yon density of the universe both during and after its reionization epo ch. (2) As a result, our previous conclusion that the observed QSOs at high redshift are not sufficient to ionize the IGM enough to satisfy the Gunn-Peterson constraint is confirmed. (3) We predict a detectable He II Gunn-Peterson effect at 304(1 + z) angstrom in the spectra of q uasars at a range of redshift z greater than or similar to 3, dependin g on the nature of the sources of IGM reionization. (4) We find, moreo ver, that a CDM model with high bias parameter b (i.e., b greater than or similar to 2) cannot account for the baryon content of the univers e at z approximately 3 observed in quasar absorption line gas unless O MEGA(B) significantly exceeds the maximum value allowed by big bang nu cleosynthesis. (5) For a CDM model with bias parameter within the allo wed range of (lower) values, the lower limit to OMEGA(B) imposed by bi g bang nucleosynthesis (OMEGA(B) h2 greater-than-or-equal-to 0.01) com bines with our results to yield the minimum IGM density for the CDM mo del. For CDM with b = 1 (COBE normalization), we find OMEGA(IGM)min(z approximately 4) congruent-to 0.02-0.03, and OMEGA(IGM)min(z approxima tely 0) congruent-to 0.005-0.03, depending upon the nature of the sour ces of IGM reionization. (6) In general, we find that self-consistent reionization of the IGM by the collapsed baryon fraction has a strong effect on the rate of collapse. (7) As a further example, we show that the feedback effect on the IGM of energy release by the collapsed bar yon fraction may explain the slow evolution of the observed comoving Q SO number density between z = 5 and z = 2, followed by the sharp decli ne after z = 2.