Hydrogen Clouds before Reionization: A Lognormal Model Approach

We study the baryonic gas clouds (the IGM) in the universe before thereionization with the lognormal model which is shown to be dynamciallylegitimate in describing the fluctuation evolution in quasilinear as well asnonlinear regimes in recent years. The probability distribution function of themass field in the LN model is long tailed and so plays an important role inrare events, such as the formation of the first generation of baryonic objects.We calculate density and velocity distributions of the IGM at very high spatialresolutions, and simulate the distributions at resolution of 0.15 kpc from z=7to 15 in the LCDM cosmological model. We performed a statistics of the hydrogenclouds including column densities, clumping factors, sizes, masses, and spatialnumber density etc. One of our goals is to identify which hydrogen clouds aregoing to collapse. By inspecting the mass density profile and the velocityprofile of clouds, we found that the velocity outflow significantly postponesthe collapsing process in less massive clouds, in spite of their masses arelarger than the Jeans mass. Consequently, only massive (> 10^5 M_sun) cloudscan form objects at higher redshift, and less massive (10^4-10^5) collapsedobjects are formed later. For example, although the mass fraction in cloudswith sizes larger than the Jeans length is already larger than 1 at z=15, thereis only a tiny fraction of mass (10^{-8}) in the clouds which are collapsed atthat time. If all the ionizing photons, and the 10^{-2} metallicity observed atlow redshift are produced by the first 1% mass of collapsed baryonic clouds,the majority of those first generation objects would not happen until z=10.Comment: Paper in AAStex, 12 figure