Gas in Simulations of High Redshift Galaxies and Minihalos

We study the gas content of halos in the early universe using high resolution hydrodynamical simulations. We extract from the simulations and also predict based on linear theory the halo mass for which the enclosed baryon fraction equals half of the mean cosmic fraction. We find a rough agreement between the simulations and the predictions, which suggests that during the high-redshift era before stellar heating, the minimum mass needed for a minihalo to keep most of its baryons throughout its formation was $\sim 3 \times 10^4$ M$_\odot$. We also carry out a detailed resolution analysis and show that in order to determine a halo's gas fraction even to 20% accuracy the halo must be resolved into at least 500 dark matter particles.