Molecular gas and star formation in an absorption-selected galaxy: Hitting the bull’s eye at z ≃ 2.46^⋆,⋆⋆

¹ Institut d’astrophysique de Paris, UMR 7095, CNRS-SU, 98bis bd Arago, 75014 Paris, France
e-mail: adarsh.ranjan@iap.fr
² Ioffe Institute of RAS, Polytekhnicheskaya 26, 194021 Saint-Petersburg, Russia
³ Raymond and Beverly Sackler School of Physics & Astronomy, Tel Aviv University, 69978 Ramat Aviv, Israel
⁴ Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, 411 007 Pune, India
⁵ Cosmic Dawn Center, Niels Bohr Institute, Copenhagen University, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁶ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
⁷ Oskar Klein Centre, Dept. of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden

Received: 17 May 2018
Accepted: 19 June 2018

Abstract

We present the detection and detailed analysis of a diffuse molecular cloud at z_abs = 2.4636 towards the quasar SDSS J 1513+0352 (z_em ≃ 2.68) observed with the X-shooter spectrograph at the Very Large Telescope. We measured very high column densities of atomic and molecular hydrogen with log N(H I, H₂) ≃ 21.8, 21.3. This is the highest H₂ column density ever measured in an intervening damped Lyman-α system but we did not detect CO, implying log N(CO)/N(H₂) < −7.8, which could be due to a low metallicity of the cloud. From the metal absorption lines, we derived the metallicity to be Z ≃ 0.15 Z_⊙ and determined the amount of dust by measuring the induced extinction of the background quasar light, A_V ≃ 0.4. We simultaneously detected Lyman-α emission at the same redshift with a centroid located at a most probable impact parameter of only ρ ≃ 1.4 kpc. We argue that the line of sight is therefore likely passing through the interstellar medium (ISM), as opposed to the circumgalactic medium (CGM), of a galaxy. The relation between the surface density of gas and that of star formation seems to follow the global empirical relation derived in the nearby Universe although our constraints on the star formation rate (SFR) and the galaxy extent remain too loose to be conclusive. We study the transition from atomic to molecular hydrogen using a theoretical description based on the microphysics of molecular hydrogen. We use the derived chemical properties of the cloud and physical conditions (T_k ≃ 90 K and n ≃ 250 cm⁻³) derived through the excitation of H₂ rotational levels and neutral carbon fine structure transitions to constrain the fundamental parameters that govern this transition. By comparing the theoretical and observed H I column densities, we are able to bring an independent constraint on the incident ultra-violet (UV) flux, which we find to be in agreement with that estimated from the observed SFR.