(1c) Hydrogen Molecules in the Evolving Diffuse IGM in a
Cold Dark Matter (CDM) Model Universe
Following the recombination epoch at a redshift z~10^3, the IGM grew
cold, dark, and mostly neutral prior to the condensation of the first bound
objects to form stars and/or quasars. The significance of H_2 molecules
as coolants in primordial composition gas led Shapiro and his collaborators
to reconsider the abundance of H_2
in the diffuse IGM within the context of the
CDM model for cosmic structure formation (
1992;
1994;
1996). As described more fully in item (2) below, detailed
numerical calculations of the thermal and ionization balance, molecule
formation and destruction, and radiative transfer in a uniform IGM of H and He
were coupled to the linearized equations for the growth of density fluctuations
in both the gaseous baryon-electron and dark matter components.
As baryons condensed out of the background IGM, they were presumed to
release ionizing radiation at a rate sufficient to account for the late-time
minimum ionization level of the surviving diffuse IGM implied by the
observed limits on the Gunn-Peterson optical depth at z<5. As in
previous studies of a uniform IGM evolving without dark matter or structure
formation, H_2 formation shortly after recombination was found to
be limited by photodestruction of H- and H_2+ by the Cosmic
Microwave Background.
For 200 < z < 600, the H_2+ process dominated the formation
of H_2, yielding a concentration <10^-6 by z~300.
By z~100, the H^- process boosted this to ~2x10^-6,
before the collapsed fraction finally released enough radiation by
z < 25 (the particular redshift depending upon the assumed amplitude
of the initial density fluctuations on subgalactic mass scales) to heat
and ionize the IGM and destroy the H_2, even before the ionization
of the IGM was complete.