= Atomic and molecular cooling thresholds for galaxy formation
{title2=$T_{\rm atomic}\sim10^4\,\mathrm K,\quad T_{\rm molecular}\sim10^2\text{--}10^3\,\mathrm K$}
Primordial <atomic line cooling> becomes effective near the temperature where electronic excitations are accessible, of order ten thousand kelvin. <Molecular hydrogen> has lower-energy transitions and can cool colder gas if it forms and survives. The corresponding <virial temperature> thresholds translate into epoch-dependent halo mass thresholds. Radiation, molecular chemistry, <metal-line cooling> and gas density qualify a single universal minimum galaxy mass.
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