2.2 The atomic-cooling stage

An atomic H-He mixture behaves adiabatically unless T is as high as 8-10 thousand degrees, when excitation of Lyman alpha by the Maxwellian tail of the electrons provides efficient cooling whose rate rises steeply with temperature.

When H₂ cooling has been quenched, primordial gas cannot therefore cool and fragment within bound systems unless their virial temperature reaches 10⁴ K. The corresponding mass is ~ 10⁸ M. Most of the UV that ionized the IGM therefore came from stars (or perhaps from accreting black holes) that formed within systems of total mass 10⁸ M.