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7.3. Cooling

In hierarchical models, structure grows by gravitational collapse of succesively larger structures. The emergence of galaxies as separate entities within this hierarchy depends on the ability of baryons within a given overdense region to cool and form stars before being subsumed into the next higher level of the hierarchy. Cooling of gas in protogalaxies is thought to be a runaway process. As soon as gas is able to cool in a galaxy halo it contracts, whereupon the cooling becomes more efficient, leading to more cooling and contraction until something (e.g. star formation) is able to put a halt the process. The virial temperatures (k Tvir approx µ mH sigma2, where µ mH is the mean molecular weight of the gas) of collapsing protogalaxies are high enough that the associated gas will be collisionally ionized and cool radiatively. In the absence of other energy sources or sources of ionization, the cooling time at radius r can be written as

Equation 12 (12)

where Lambda(T) is the cooling rate due to bremsstrahlung, recombination, and collisionally excited line emission, and rho (r) is the density at r (e.g. White and Frenk 1991; Cole et al. 1994).

For dwarf galaxy halos collapsing at z approx 3-10, the cooling time is short compared to the free-fall time, so cooling should be efficient. Unless this cooling is suppressed, most of the baryonic material in the universe would collapse into dwarf-galaxy size objects. This is the ``overcooling problem'' noted by Cole (1991) and others. As dwarf galaxies do not appear to be so overwhelming abundant, some mechanism to counteract cooling is needed. Plausible mechanisms involve both internal and external agents.