Dark satellites and cosmic reionization

A possible explanation of the present discrepancy between the abundance of galactic subhaloes predicted by N-Body simulations with those observed in the Local Group is presented. We study the impact of an early reionization on the baryonic component of the Universe using SPH simulations on group and galactic scales. We use a simplified model for reionization described as an instantaneous increment of the IGM temperature (with 10⁵ < TIGM < 10⁶ K) at a given redshift (zr = 8). We find that a key role is played by compton cooling (interaction between hot electrons and the CMB photons); at high redshift (zr > 10) this cooling is very efficient and it is able to counteract any heating of the gas within few Myrs (˜ 70). This means that a late reionization is needed (zr < 9) to sufficiently reduce the number of luminous dwarf satellites around our Galaxy. For a reionization redshift zr = 8 and a reionization temperature of TIGM ˜ 10⁵ K we are able to reproduce the observed number of Local Group dwarf galaxies in our simulations.