The formation of the virial shock in galactic halos
The main finding of the thesis is that the development of a global shock in response to the cosmological gas accretion onto dark-matter halos introduces a characteristic mass scale, about 10^12 solar masses. This transition plays a major role in determining the properties of galaxies. For halos below this critical mass, the inflowing gas is not heated by a shock at the virial radius, and can stream cold into the inner halo where it builds a gaseous disk and form stars there. In halos above the threshold mass, the infalling gas is heated and halted by a virial shock, which suppresses the gas supply to the disk. The formation of virial shocks in halos is investigated by analyzing the gravitational stability of post gas in the presence of abrupt cooling. We find that post-shock gas is unable to support the virial shock if the compression time is larger than the cooling time. This criterion is compared with spherical Lagrangian hydrodynamic calculations of halo formation performed by a code written and validated in this work. The simulations yield results similar to the analytic criterion. Finally, the microscopic stability criterion is converted into a cosmological criterion by using the top hat and virial relations, resulting in the critical halo mass.