The formation and gas content of high-redshift galaxies and minihaloes

We investigate the suppression of the baryon density fluctuations compared to the dark matter in the linear regime. Previous calculations predict that the suppression occurs up to a characteristic mass scale of ~10⁶M_solar, which suggests that pressure has a central role in determining the properties of the first luminous objects at early times. We show that the expected characteristic mass scale is in fact substantially lower (by a factor of ~3-10, depending on redshift), and thus the effect of baryonic pressure on the formation of galaxies up to reionization is only moderate. This reduction in the characteristic mass scale is a result of including the fact that the baryons have smoother initial conditions than the dark matter. This smoothing is due to the aftereffects of the coupling with the radiation before recombination. This yields lower pressure gradients than previous estimations in the period from cosmic recombination to z ~ 200, after which the gas cooled down adiabatically and the pressure dropped. At z ~ 10 the suppression of the baryon fluctuations is still sensitive to the history of pressure in this high-redshift era. We calculate the fraction of the cosmic gas that is in minihaloes and find that it is substantially higher than would be expected with the previously estimated characteristic mass. Expanding our investigation to the non-linear regime, we calculate in detail the spherical collapse of high-redshift objects in a Λ cold dark matter universe. We include the gravitational contributions of the baryons and radiation and the memory of their kinematic coupling before recombination. We use our results to predict a more accurate halo mass function as a function of redshift.