(2c) Reionization Feedback and The Photoevaporation
of Intergalactic Clouds
Energy released by a small fraction of the baryons in the universe, which
condensed out while the IGM was cold, dark, and neutral, reheated and reionized
it before redshift 5, exposing gas clouds within it to the glare of ionizing
radiation. The first gas dynamical simulations of the photoevaporation of an
intergalactic cloud overtaken by the ionization front which expands into the
IGM when a quasar or stellar source turns on, including radiative transfer,
have
been performed by Shapiro, Raga, and Mellema
(1997;
1998:
1999).
These results, by a 2D, axisymmetric, Eulerian hydro code with adaptive
mesh refinement and a Riemann solver, which includes the nonequilibrium
ionization of H, He, C, N, O, and S and the transfer of ionizing radiation,
demonstrate the phenomena of ionization-front ("I-front") trapping inside
density enhancements or clouds which condense out of the IGM, including
the transition from an R-type to a D-type I-front preceded by a shock,
a supersonic evaporative wind, the rocket effect, and their observable
consequences. They suggest that such hitherto neglected effects
were widespread during the reionization epoch and may have contributed
to the so-called Proximity Effect, possibly requiring a revision of the
interpretation of that effect entirely as a measure of the relative intensity
of the metagalactic ionizing background and that of the nearby quasar.
These calculations represent the first attempts to incorporate realistic
radiative transfer in numerical gas dynamics applied to cosmology.