Probing the Conditions for the Atomic-to-Molecular Transition in the Interstellar Medium

We examine the physical conditions required for the formation of H₂ in the solar neighborhood by comparing H I emission and absorption spectra toward 58 lines of sight at b < −5^∘ to CO(1-0) and dust data. Our analysis of CO-associated cold and warm neutral medium (CNM and WNM) shows that the formation of CO-traced molecular gas is favored in regions with high column densities where the CNM becomes colder and more abundant. In addition, our comparison to the one-dimensional steady-state H I-to-H₂ transition model of Bialy et al. (2016) suggests that only a small fraction of the clumpy CNM participates in the formation of CO-traced molecular gas. Another possible interpretation would be that missing physical and chemical processes in the model could play an important role in H₂ formation.