Hydrogen Clouds before Reionization: A Lognormal Model Approach

Abstract

We study the baryonic gas clouds (the intergalactic medium) in the universe before reionization with the lognormal (LN) model, which has been shown to be dynamically legitimate in describing the fluctuation evolution in quasilinear as well as nonlinear regimes in recent years. The probability distribution function of the mass field in the LN model is long-tailed and so plays an important role in rare events, such as the formation of the first generation of baryonic objects. Since in this model the nonlinear field is directly mapped from the corresponding linear field, we can calculate the density and velocity distributions of the intergalactic medium at very high spatial resolutions. We simulate the distributions at a resolution of 0.15 kpc from z = 7 to 15 in the low-density cold dark matter cosmological model. We analyze statistics on the hydrogen clouds at high redshifts, including column densities, clumping factors, sizes, masses, and spatial number densities. One of our goals is to identify which hydrogen clouds are going to collapse. By inspecting the mass density profiles and the velocity profiles of clouds, we find that the velocity outflow significantly postpones the collapsing process in less massive clouds, even when their masses are larger than the Jeans mass. This indicates that the formation of collapsed clouds with small mass at high redshift is substantially suppressed. Consequently, only massive (>105 M☉) clouds can form objects at higher redshifts, and less massive (104-105 M☉) collapsed objects are formed later. Although the mass fraction in clouds with sizes larger than the Jeans length is already larger than 1% at z = 15, there is only a tiny fraction of mass (10-8) in collapsed clouds. If all the ionizing photons and the ~10-2 metallicity observed at low redshift are produced by the first 1% of the mass of collapsed baryonic clouds, the majority of that first generation of objects would be occurring not much earlier than z = 10.