Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Second-Generation Objects in the Universe: Radiative Cooling and
  Collapse of Halos with Virial Temperatures Above 10^4 Kelvin   The first generation of protogalaxies likely formed out of primordial gas via
H2-cooling in cosmological minihalos with virial temperatures of a few 1000K.
However, their abundance is likely to have been severely limited by feedback
processes which suppressed H2 formation. The formation of the protogalaxies
responsible for reionization and metal-enrichment of the intergalactic medium,
then had to await the collapse of larger halos. Here we investigate the
radiative cooling and collapse of gas in halos with virial temperatures Tvir >
10^4K. In these halos, efficient atomic line radiation allows rapid cooling of
the gas to 8000 K; subsequently the gas can contract nearly isothermally at
this temperature. Without an additional coolant, the gas would likely settle
into a locally gravitationally stable disk; only disks with unusually low spin
would be unstable. However, we find that the initial atomic line cooling leaves
a large, out-of-equilibrium residual free electron fraction. This allows the
molecular fraction to build up to a universal value of about x(H2) = 10^-3,
almost independently of initial density and temperature. We show that this is a
non--equilibrium freezeout value that can be understood in terms of timescale
arguments. Furthermore, unlike in less massive halos, H2 formation is largely
impervious to feedback from external UV fields, due to the high initial
densities achieved by atomic cooling. The H2 molecules cool the gas further to
about 100K, and allow the gas to fragment on scales of a few 100 Msun. We
investigate the importance of various feedback effects such as
H2-photodissociation from internal UV fields and radiation pressure due to
Ly-alpha photon trapping, which are likely to regulate the efficiency of star
formation.

31
The categories of this paper are: ao-sci
4
The categories of this paper are: cond-mat
0
The categories of this paper are: hep-ph
2
The categories of this paper are: math
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The categories of this paper are: cmp-lg
14
The categories of this paper are: math-ph
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The categories of this paper are: eess
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The categories of this paper are: q-alg
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The categories of this paper are: atom-ph
35
The categories of this paper are: mtrl-th
5
The categories of this paper are: gr-qc
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The categories of this paper are: solv-int
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The categories of this paper are: chem-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: q-fin
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The categories of this paper are: dg-ga