= Cooling criterion for galaxy formation
{title2=$t_{\rm cool}\lesssim t_{\rm dyn}$}
Gas in a collapsing halo is heated toward a <virial temperature>. If its cooling time is short compared with the dynamical time, thermal support is removed and rapid condensation can form a galaxy. If cooling is slower, a hot atmosphere can persist or condense only gradually. This criterion links baryonic galaxy formation to the dark-matter assembly hierarchy without making every halo into one luminous object.
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