Figure 2
Top panel : thermal evolution of primordial gas as it collapses over many orders of magnitude in density. The labels denotes various milestones in the collapse: heating to the virial temperature of the halo (A), runaway cooling via H2 line emission (B), cooling to the minimum temperature of \({\simeq}200\mbox { K}\) (C), onset of three-body H2 formation (D), gas becomes optically thick to H2 line emission (E), onset of collision-induced emission (F), and collisional dissociation of H2 (G). Bottom panel: H2 fraction versus density. The H2 fraction increases from its cosmological abundance of ≃10−6 to ≃10−3 via associative detachment of H and H−. Following an extended plateau where the H2 fraction remains nearly constant, the cloud becomes fully molecular once three-body reactions set in. Adapted from Yoshida et al. (2006).