2015A&A...575L...8B

Chlorine and molecular hydrogen are known to be tightly linked together in the cold phase of the local interstellar medium through rapid chemical reactions. We present here the first systematic study of this relation at high redshifts using H₂-bearing damped Lyα systems (DLAs) detected along quasar lines of sight. Using high-resolution spectroscopic data from VLT/UVES and Keck/HIRES, we report the detection of Cli in 9 DLAs (including five new detections) out of 18 high-z DLAs with N(H₂)≥10^17.3cm^–2 (including a new H₂ detection at z=3.09145 towards J2100-0641) and present upper limits for the remaining 9 systems. We find a ∼5σ correlation between N(Cli) and N(H₂) with only ∼0.2 dex dispersion over the range 18.1<logN(H₂)<20.1, thus probing column densities 10 times lower than those seen towards nearby stars, roughly following the relation N(ClI)≃1.5x10^–6xN(H₂). This relation between column densities is surprisingly the same at low and high redshift suggesting that the physical and chemical conditions are similar for a given H₂ (or ClI) column density. In turn, the N(ClI)/N(H₂) ratio is found to be uncorrelated with the overall metallicity in the DLA. Our results confirm that neutral chlorine is an excellent tracer of molecule-rich gas and show that the molecular fraction and/or metallicity in the H₂-bearing component of DLA could possibly be much higher than the line-of-sight average values usually measured in DLAs.