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[[File: Cooling_curve.jpg | cooling rate]]

credit: http://ned.ipac.caltech.edu/level5/Sept06/Loeb/Loeb5.html

Cooling rates as a function of temperature for a primordial gas composed of atomic hydrogen and helium, as well as molecular hydrogen, in the absence of any external radiation. We assume a hydrogen number density nH = 0.045 cm-3, corresponding to the mean density of virialized halos at z = 10. The plotted quantity Lambda / nH2 is roughly independent of density (unless nH > 10 cm-3), where Lambda is the volume cooling rate (in erg/sec/cm3). The solid line shows the cooling curve for an atomic gas, with the characteristic peaks due to collisional excitation of H1 and He2. The dashed line shows the additional contribution of molecular cooling, assuming a molecular abundance equal to 1% of nH.