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Fig. 8.

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Column density of molecular hydrogen vs total neutral hydrogen column density N(H I) = N(H I) + 2N(H2) for our sample of proximate H2 absorbers and z > 2 intervening strong H2-bearing DLAs from Balashev et al. (2019). For a fair comparison, we removed systems that may potentially be proximate, and kept only systems with a conservative cut of c (zabs − zQSO)/(1 + zQSO) <  − 10 000 km s−1. Our selection of proximate H2 systems based on strong H2 lines directly seen in the low-resolution SDSS spectra naturally skews our sample to high H2 columns. The horizontal blue line shows the approximate and empirical lower limit of at log N(H2) > 18.5 for the present sample. The top histograms show the N(H) distributions for the proximate (blue) and intervening (green) systems with N(H2) > 18.5. Finally, the different lines correspond to different averaged molecular fractions fH2 ≡ 2N(H2)/2N(H2) + N(H I)).

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