Display, Wednesday, January 10, 2001, 9:30am-7:00pm, Exhibit Hall

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*T. Jena, M.L. Norman (UCSD), T. Abel (CFA), G. Bryan (MIT)*

The first baryonic bound objects to form in hierarchical
models of structure formation are low mass systems (10^{3}
M_{\odot} \le M_{tot} \le 10^{6} M_{\odot}) composed of cold
dark matter and primordial gas. In previous work we have
studied how a single primordial halo cools and collapses to
form the first stars in the universe. Halos insufficiently
massive to form stars pervade the universe at high redshift
and may have observable consequences at lower redshifts.
Here we present the results of hydrodynamic cosmological
simulations studying the statistical properties of the first
bound objects (FBO) to form in a standard cold dark matter
universe. We conduct a numerical resolution study using
adaptive mesh refinement with dark matter particle masses of
8M_{\odot}, 64M_{\odot}, and 512M_{\odot} in a
simulation volume of 128 comoving kpc on a side. We find
that the number of collapsed objects is sensitive to mass
resolution, as expected, with over 1000 halos present in our
highest resolution simulation. A statistical analysis of
this sample yields the following results: The dark matter
halo population is in full agreement with a Press-Schecter
analysis for {\delta}_{c} = 1.68 over the range of halos
which are well resolved in our simulations (10^{3} M_{\odot}
< M_{vir} < 10^{6} M_{\odot}). These halos are found to have
a spin parameter \lambda distribution which is identical
to N-body simulations of galaxy mass halos; <\lambda> =
0.04 and \lambda was found to be uncorrelated with the
halo mass. The baryonic properties of these systems are more
interesting. We find that pressure effects depletes the
baryons relative to the dark matter by a factor of ~10
at M_{vir} = 10^{3} M_{\odot}, diminishing smoothly to unity
for M_{vir} ~10^{5} M_{\odot}. In addition, we find that
the gas angular momentum is correlated with the dark matter
angular momentum with a large scatter, with a mean value
lower by an order of magnitude. Finally, we consider how
baryons trapped in FBOs would effect cosmological
reionization. We find that the clumping factor C_{bb}
\equiv <{\rho}^{2}>/{<\rho>}^{2} > 10 at z = 20, implying a
lower reionization redshift z_{reion} < 7 than obtained by
Gnedin & Ostriker (1997; ApJ 486, 581) with lower
resolution simulations.

This research was partially supported by NSF grant AST-9803137. Simulations were carried out on the SGI/Cray Origin2000 system at the NCSA.

The author(s) of this abstract have provided an email address for comments about the abstract: tjena@physics.ucsd.edu