Neutral chlorine and molecular hydrogen at high redshift

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 H-2-bearing damped Ly alpha 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(H-2) >= 10(17.3) cm(-2) (including a new H-2 detection at z = 3.09145 towards J 2100-0641) and present upper limits for the remaining 9 systems. We find a similar to 5 sigma correlation between N(ClI) and N(H-2) with only similar to 0.2 dex dispersion over the range 18.1 < log N(H-2) < 20.1, thus probing column densities 10 times lower than those seen towards nearby stars, roughly following the relation N(ClI) approximate to 1.5 x 10(-6) x N(H-2). 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 H-2 (or ClI) column density. In turn, the N(ClI)/N(H-2) 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 H-2-bearing component of DLA could possibly be much higher than the line-of-sight average values usually measured in DLAs.