Next Contents Previous

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 Msun. 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 gtapprox 108 Msun.