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 $5 10 M ⊙$ . 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 $108M ⊙$ . This reheating could affect the subsequent development of structures such as galaxies and the observed Ly $α$ clouds.