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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 (103 M\odot \le Mtot \le 106 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 (103 M\odot < Mvir < 106 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 Mvir = 103 M\odot, diminishing smoothly to unity for Mvir ~105 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 Cbb \equiv <{\rho}2>/{<\rho>}2 > 10 at z = 20, implying a lower reionization redshift zreion < 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