Neutral chlorine and molecular hydrogen at high redshift

Abstract

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 5 new detections) out of 18 high-$z$ DLAs with $N($H${}_2)\ge 10^{17.3}$cm${}^{-2}$ (including a new H${}_2$ detection at $z=3.09145$ towards J2100$-$0641) and present upper limits for the remaining 9 systems. We find a $\sim$5$\sigma$ correlation between $N$(ClI) and $N$(H${}_2$) with only $\sim$0.2dex dispersion over the range 18.1$<$log$N$(H${}_2$)$<$20.1, thus probing column densities 10 times lower those seen towards nearby stars, roughly following the relation $N$(ClI$)\approx 1.5\times 10^{-6}\times N($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 $N({Cl$\,$I})/N({\rm H_2})$ 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 or/and 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.

5 pages, 3 figures, Accepted for publication in A&A Letters

References in corpus (12)

• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• On the (Non)Evolution of HI Disks over Cosmic Time
• Metallicity Evolution of Damped Lyman-alpha Systems out to z~5
• Molecular hydrogen in high-redshift Damped Lyman-alpha systems: The VLT/UVES database
• Metallicity as a criterion to select H2 bearing Damped Lyman-alpha systems
• Excitation mechanisms in newly discovered H2-bearing Damped Lyman-alpha clouds: systems with low molecular fractions
• Physical conditions in the neutral interstellar medium at z=2.43 toward Q2348-011
• Molecular Hydrogen in a Damped Lyman-alpha System at z_abs=4.224
• Molecular hydrogen absorption systems in Sloan Digital Sky Survey
• HD/H2 Molecular Clouds in the Early Universe: The Problem of Primordial Deuterium
• Variations in the D/H ratio of extended sightlines from FUSE observations
• Chlorine in the Galactic ISM: Revised f-values with FUSE and STIS

Cited by in corpus (13)

• Discovery of a Perseus-like cloud in the early Universe: HI-to-H2 transition, carbon monoxide and small dust grains at zabs=2.53 towards the quasar J0000+0048
• CO-dark molecular gas at high redshift: very large H${}_2$ content and high pressure in a low metallicity damped Lyman-alpha system
• VLT/UVES observations of extremely strong intervening damped Lyman-alpha systems: Molecular hydrogen and excited carbon, oxygen and silicon at log N(HI)=22.4
• X-shooter observations of strong H${}_2$-bearing DLAs at high redshift
• The elusive HI-> H2 transition in high-z damped Lyman-alpha systems
• Quasar 2175 Å dust absorbers II: Correlation analysis and relationship with other absorption line systems
• Constraint on a varying proton-to-electron mass ratio from H2 and HD absorption at z = 2.34
• Physical conditions in diffuse interstellar medium of local and high redshift galaxies: measurements based on excitation of H${}_2$ rotational and CI fine-structure levels
• Down-the-barrel observations of a multiphase quasar outflow at high redshift: VLT/X-shooter spectroscopy of the proximate molecular absorber at z=2.631 towards SDSS J001514+184212
• A Reexamination of Phosphorus and Chlorine Depletions in the Diffuse Interstellar Medium
• Proximate molecular quasar absorbers: Chemical enrichment and kinematics of the neutral gas
• Multi-phase gas properties of extremely strong intervening DLAs towards quasars
• Neutral carbon in diffuse interstellar medium: abundance matching with H2 for DLAs at high redshifts