The molecular to atomic ratio a), the molecular fraction b), and the H₂ and H₂ +Hi depletion timescale (c) and d), respectively) as a function of metallicity. Symbols are as in Fig. 8. Black and gray dashed lines show linear best fits to all data points and only secure detections, respectively. Solid, dotted, and dashed magenta lines in panel c show model predictions by Krumholz et al. (2011) for different values of Σ_{H2 + HI} (80, 20 and 5 M_⊙ pc^-2, respectively).