In addition, it is well known that gas which
cools to
through hydrogen line cooling will likely cool faster than it
can recombine. This nonequilibrium cooling increases the number
of electrons and ions (compared to the equilibrium case) which,
in turn, increases the concentrations of
and
, the intermediaries that produce hydrogen molecules
. If large concentrations of molecules form, excitations of the
vibrational/rotational modes of the molecules can efficiently
cool the gas to well below
, the minimum temperature expected from atomic hydrogen line
cooling. Because the gas cools isobarically, the reduction in
temperature results in an even greater reduction in the Jeans
mass, and the bound objects which form from the fragmentation of
cooled cosmological sheets may be associated with massive stars
or star clusters. Anninos and Norman
[18]
have carried out 1D and 2D high resolution numerical
calculations to investigate the role of hydrogen molecules in the
cooling instability and fragmentation of cosmological sheets,
considering the collapse of perturbation wavelengths from
to
. They find that for the more energetic (long wavelength) cases,
the mass fraction of hydrogen molecules reaches
, which cools the gas to
eV and results in a fragmentation scale of
. This represents reductions of 50 and
in temperature and Jeans mass respectively when compared, as in
Figure
12
, to the equivalent case in which hydrogen molecules were
neglected.
However, the above calculations neglected
important interactions arising from self-consistent treatments of
radiation fields with ionizing and photo-dissociating photons and
self-shielding effects. Susa and Umemura
[153]
studied the thermal history and hydrodynamical collapse of
pancakes in a UV background radiation field. They solve the
radiative transfer of photons together with the hydrodynamics and
chemistry of atomic and molecular hydrogen species. Although
their simulations were restricted to one-dimensional plane
parallel symmetry, they suggest a classification scheme
distinguishing different dynamical behavior and galaxy formation
scenarios based on the UV background radiation level and a
critical mass corresponding to
density fluctuations in a standard CDM cosmology. These level
parameters distinguish galaxy formation scenarios as they
determine the local thermodynamics, the rate of
line emissions and cooling, the amount of starburst activity,
and the rate and mechanism of cloud collapse.
![]() |
http://www.livingreviews.org/lrr-2001-2 | © Max Planck Society and
the author(s)
Problems/comments to |