(2b) Reionization in a Cold Dark Matter Universe: The Feedback
of Galaxy Formation on the Intergalactic Medium
Consequences
Shapiro, Giroux, and Babul (1994, "Paper I") [i.e. Paper I actually
preceded Paper II described in the paragraph above], incorporated the numerical
method of the coarse-grained-average description of
the IGM developed and tested
in Paper II in a calculation of the feedback effects of reionization on the IGM
and on cosmic structure formation in the Cold Dark Matter ("CDM") model.
Paper I coupled the IGM code of Paper II to a solution of the linearized
equations for the growth of density fluctuations in both the gaseous
baryon-electron and dark
matter components in a CDM universe, in order
to calculate the rate of nonlinear
collapse of baryons out of the background IGM. The collapsed baryon fraction,
calculated using a modified form of the Press-Schecter approximation which
took account of the "Jeans mass filtering" of baryonic fluctuations, was
postulated to be a source of ionizing energy release back into the IGM, while
the mean density of the IGM was assumed to be that of the uncollapsed fraction.
These calculations showed that the IGM in a CDM model must have contained a
substantial fraction of the total baryon density of the universe both during
and after its reionization epoch, a view now supported by more recent gas
dynamical simulations. We predicted a detectable He II Gunn-Peterson effect
at redshifts of order 3 and above, a prediction which preceded the UV detectionsof He II Lyman alpha opacity at these redshifts by HST and HUT. We found that
the observed quasars were not sufficient to photoionize the IGM at the level
observed at high redshift, so additional sources such as massive stars were
required. The latter were shown to generate a substantial metallicity at high
redshift, consistent with the subsequent widespread detection of metals in the
Lyman alpha forest. These calculations demonstrated that heating the IGM by
the energy release responsible for its reionization at high redshift raised the
Jeans mass in the IGM and thereby damped the formation of baryonic structure.
This effect was shown to rule out the kind of low-amplitude CDM models which
had previously been considered "standard CDM" before
COBE, since the collapsed
baryon fraction at z=3 in such models failed to account for the observed
baryon content of the Damped Lyman Alpha and Lyman Limit System quasar
absorption-line gas.
As summarized in the review by
Shapiro (1995), these calculations were
followed up by high resolution numerical gas dynamical simulations of the
evolution of the IGM and the formation of structure in the gas and dark
matter in CDM and CHDM (Cold + Hot Dark Matter) models, by the new ASPH method
developed by Shapiro and his collaborators, described below in item (3). The
results confirm the importance of the feedback effects of energy release and
global reheating on cosmic structure formation in the baryonic component.
These ASPH simulations, summarized also in
Shapiro and Martel 1995, and detailed in
Shapiro and Martel (2001, in preparation),
are mentioned in item (3)(b) below.