= Upper galaxy mass from gas cooling
{title2=$M_{\rm cool}\text{ of order a galactic mass}$}
In simple hierarchical models, increasing system mass raises the <virial temperature> and can carry primordial gas beyond its efficient atomic line-cooling interval. At assembly densities, the cooling-time constraint then distinguishes galaxy-sized condensations from larger hot groups or clusters. The resulting characteristic mass is approximate, depends on composition and structure, and does not prohibit subsequent mergers of already formed stellar systems.
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