From the relative excitation of fine-structure levels of C I, we constrain the hydrogen volumetric densities to lie in the range of 40 − 110 cm −3. Based on the excitation of low rotational levels of H 2, we constrain the temperature of the cold gas phase to be T = 109 ± 20 and T = 89 ± 25 K for the two lines of sight. The metallicity inferred from sulphur is consistent with solar metallicity for both sightlines: A = 0.0 ± 0.1 and B = −0.1 ± 0.1. We measure the column densities of H I to be log N(H I) = 20.27 ± 0.02 and 20.34 ± 0.05 and those of H 2 to be log N(H 2) = 19.7 ± 0.1 and 19.9 ± 0.2. We observe absorption lines from neutral carbon and H 2 along both lines of sight, indicating that cold gas is present on scales larger than d abs. The physical separation of the two lines of sight at the absorber redshift is d abs = 0.7 kpc according to our lens model. We present a study of cold gas absorption from a damped Lyman- α absorber (DLA) at redshift z abs = 1.946 toward two lensed images of the quasar J144254.78+405535.5 at redshift z QSO = 2.590. 26, Saint Petersburg, 194021, Russiaġ1 Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerlandġ2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USAġ3 Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD, 21218, USAġ4 European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile Boissé 1ġ Institut d’Astrophysique de Paris, CNRS-SU, UMR7095, 98bis bd Arago, 75014 Paris, FranceĮ-mail: Department of Chemistry and Physics, Saint Michael’s College, One Winooski Park, Colchester, VT, 05439, USAģ Centre for Extragalactic Astronomy, Durham University, South Road, Durham, DH1 3LE, UKĤ Institute for Computational Cosmology, Durham University, South Road, Durham, DH1 3LE, UKĥ The Cosmic Dawn Center, Niels Bohr Institute, University of Copenhagen, Juliane Maries Copenhagen Ø, DenmarkĦ Department of Astronomy and Astrophysics, University of California, 1156 High Street, Santa Cruz, CA, 95064, USAħ University of California Observatories, Lick Observatory, 1156 High Street, Santa Cruz, CA, 95064, USAĨ Department of Physics, Broida Hall, University of California, Santa Barbara, CA, 93106, USAĩ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germanyġ0 Ioffe Institute, Polytechnicheskaya ul. Astronomical objects: linking to databases.Including author names using non-Roman alphabets.Suggested resources for more tips on language editing in the sciences Punctuation and style concerns regarding equations, figures, tables, and footnotes Significant progress is expected to come through improved analysis techniques, increases in the number of known high-redshift quasars from optical and infrared sky surveys, large gains in sensitivity from next-generation observing facilities, and synergies with other probes of the reionization era. Along with other probes of the high-redshift Universe, absorption line data are consistent with a relatively late end to reionization ($5.5 \lesssim z \lesssim 7$) however the constraints are still fairly weak. Finally, we review constraints from the Ly$\alpha$ forest and quasar near zones on the timing of reionization. A substantial fraction of metal absorbers at these redshifts may trace relatively low-mass galaxies. Current observations suggest a buildup of metals in the circumgalactic environments of galaxies over $z \sim 6$ to 5, although changes in ionization will also affect the evolution of metal line properties. We then discuss insights from metal absorption lines into reionization-era galaxies and their surroundings. Critically, these measurements reflect the escaping ionizing radiation from all galaxies, including those too faint to detect directly. We first describe how the Ly$\alpha$ forest is used to determine the intensity of the ionizing ultraviolet background and the global ionizing emissivity budget. In this review we examine the multiple ways in which absorption lines trace the connection between galaxies and the IGM near the reionization epoch. Towards this goal, quasar absorption lines play a unique role by probing the properties of diffuse gas on galactic and intergalactic scales. Determining when and how the first galaxies reionized the intergalactic medium (IGM) promises to shed light on both the nature of the first objects and the cosmic history of baryons.
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