Dissecting cold gas in a high-redshift galaxy using a lensed background quasar^⋆

¹ Institut d’Astrophysique de Paris, CNRS-SU, UMR7095, 98bis bd Arago, 75014 Paris, France
e-mail: jens-kristian.krogager@iap.fr, krogager@iap.fr
² Department of Chemistry and Physics, Saint Michael’s College, One Winooski Park, Colchester, VT, 05439, USA
³ Centre for Extragalactic Astronomy, Durham University, South Road, Durham, DH1 3LE, UK
⁴ Institute for Computational Cosmology, Durham University, South Road, Durham, DH1 3LE, UK
⁵ The Cosmic Dawn Center, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁶ Department of Astronomy and Astrophysics, University of California, 1156 High Street, Santa Cruz, CA, 95064, USA
⁷ University of California Observatories, Lick Observatory, 1156 High Street, Santa Cruz, CA, 95064, USA
⁸ Department of Physics, Broida Hall, University of California, Santa Barbara, CA, 93106, USA
⁹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁰ Ioffe Institute, Polytechnicheskaya ul. 26, Saint Petersburg, 194021, Russia
¹¹ Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
¹² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA
¹³ Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD, 21218, USA
¹⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile

Received: 11 June 2018
Accepted: 31 August 2018

Abstract

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. 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 observe absorption lines from neutral carbon and H₂ along both lines of sight, indicating that cold gas is present on scales larger than d_abs. We measure the column densities of H I to be log N(HI) = 20.27 ± 0.02 and 20.34 ± 0.05 and those of H₂ to be log N(H₂) = 19.7 ± 0.1 and 19.9 ± 0.2. The metallicity inferred from sulphur is consistent with solar metallicity for both sightlines: [S/H]_A = 0.0 ± 0.1 and [S/H]_B = −0.1 ± 0.1. Based on the excitation of low rotational levels of H₂, 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. 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 ratio of observed column density and volumetric density, we infer the average individual “cloud” size along the line of sight to be l ≈ 0.1 pc. Using the transverse line-of-sight separation of 0.7 kpc together with the individual cloud size, we are able to place an upper limit to the volume filling factor of cold gas of f_vol < 0.1%. Nonetheless, the projected covering fraction of cold gas must be large (close to unity) over scales of a few kpc in order to explain the presence of cold gas in both lines of sight. Compared to the typical extent of DLAs (∼10 − 30 kpc), this is consistent with the relative incidence rate of C I absorbers and DLAs.