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Context. Molecular hydrogen in the interstellar medium (ISM) of high-redshift galaxies can be detected directly from its UV absorption imprinted in the spectrum of background quasars. Associated absorption from H and metals allow for the study of the chemical enrichment of the gas, while the analysis of excited species and molecules make it possible to infer the physical state of the ISM gas. In addition, given the numerous H 2 lines usually detected, these absorption systems are unique tools to constrain the cosmological variation of the proton-to-electron mass ratio, . Aims. We intend to study the chemical and physical state of the gas in the H 2 -bearing cloud at z abs = 2.658601 towards the quasar Q J 0643-5041 (z em = 3.09) and to derive a useful constraint on the variation of . Methods. We use high signal-to-noise ratio, high-resolution VLT-UVES data of Q J 0643-5041 amounting to a total of more than 23 h exposure time and fit the H , metals, and H 2 absorption features with multiple-component Voigt profiles. We study the relative populations of H 2 rotational levels and the fine-structure excitation of neutral carbon to determine the physical conditions in the H 2 -bearing cloud.
Vásquez et al. (Thu,) studied this question.