Figure 2

Figure 2: Schematic depicting the general sequence of events in the post-recombination Universe. The solid and dotted lines potentially track the Jeans mass of the average baryonic gas component from the recombination epoch at z ~ 103 to the current time. A residual ionization fraction of nH+/nH ~ 10- 4 following recombination allows for Compton interactions with photons to z ~ 200 , during which the Jeans mass remains constant at 5 10 Mo . . The Jeans mass then decreases as the Universe expands adiabatically until the first collapsed structures form sufficient amounts of hydrogen molecules to trigger a cooling instability and produce pop  III stars at z ~ 20 . Star formation activity can then reheat the Universe and raise the mean Jeans mass to above 108M o. . This reheating could affect the subsequent development of structures such as galaxies and the observed Ly a clouds.