Gas cooling

t_cool(r)=1.5n(r)T/n_e²(r)(T),

where n(r) and n_e(r) are the number density profiles of the average baryonic particles and electrons, respectively, T is the virial temperature, and is the cooling function which depends on the chemical composition of the gas. It has been assumed here that the baryonic gas is well mixed with the CDM and relaxes to the same density distribution as the CDM. Equating the cooling time with the age of the universe defines a cooling radius r_cool. Gas within r_cool can cool and form stars at that particular time if r_cool < r_v, the virial radius usually defined as the region within which the mean overdensity is 200.

For gas collapsing onto a disk, the collapse stops when rotational equilibrium is established. The length scale for the exponential disk is r₀~fmin(r_v, r_cool), where is the spin parameter, f is a numerical factor, which is unity for an isothermal halo when gravity from the disk can be negliected. See Mo et al. (1998) for more details.

References page
Introduction page