The central panel shows metallicity versus hydrogen column density for QSO and GRB DLAs in which H₂ was extensively searched for. Circles are QSO-DLAs, where red and yellow circles correspond to DLAs with and without significant molecular hydrogen. Blue data are GRB-DLAs and the blue stars are GRB 120815A and GRB 080607, where H₂ has been detected. The grey background shading indicates the parameter space where many of the more dust-extinguished GRB-DLAs are located (see details in Sect. 4.3.3). The upper and right panels illustrate the respective normalized histograms. Blue hatched histogram are GRBs including both H₂-poor and H₂-rich sightlines due to low number statistics. Red-filled and yellow histograms are QSO-DLAs with and without molecular hydrogen, respectively. Data for QSOs are from Noterdaeme et al. (2008); Fynbo et al. (2010, 2011); Guimarães et al. (2012), data from GRBs from Ledoux et al. (2009); Prochaska et al. (2009); D’Elia et al. (2010).