Galaxy counterparts of intervening high-z sub-DLAs/DLAs and Mg ii absorbers towards gamma-ray bursts^⋆

¹ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavík, Iceland
e-mail: sts30@hi.is
² Dark Cosmology Centre, Juliane Maries Vej 30, 2100 København Ø, Denmark
³ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁴ European Southern Observatory, Alonso de CÃşrdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
⁵ Department of Astronomy, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637, USA
⁶ Department of Astronomy, California Institute of Technology, MC 249-17, 1200 East California Blvd., Pasadena, CA 91125, USA
⁷ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
⁸ Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
⁹ Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia, V8P 1A1, Canada
¹⁰ European Southern Observatory, Karl-Schwarzschildstrasse 2, 85748, Garching, Germany
¹¹ UCO/Lick Observatory, University of California, 1156 High Street, Santa Cruz, CA 95064, USA
¹² Centre for Astrophysics Research, University of Hertfordshire, Hatfield, Herts AL10 9AB, UK
¹³ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹⁴ Institut d’Astrophysique et Géophysique, Université de Liège, Allée du 6 Août, 17 Bâtiment B5C, 4000 Liège, Belgium

Received: 14 December 2011
Accepted: 24 July 2012

Abstract

We present the first search for galaxy counterparts of intervening high-z (2 < z < 3.6) sub-damped Lyα absorbers (sub-DLAs) and DLAs towards gamma-ray bursts (GRBs). Our final sample comprises five intervening sub-DLAs and DLAs in four GRB fields. To identify candidate galaxy counterparts of the absorbers we used deep optical- and near-infrared imaging, and low-, mid- and high-resolution spectroscopy acquired with 6-m to 10-m class telescopes, the Hubble and the Spitzer Space Telescopes. Furthermore, we used the spectroscopic information and spectral-energy-distribution fitting techniques to study them in detail. Our main result is the detection and spectroscopic confirmation of the galaxy counterpart of the intervening DLA at z = 3.096 in the field of GRB 070721B (z_GRB = 3.6298) as proposed by other authors. We also identify good candidates for the galaxy counterparts of the two strong Mg ii absorbers at z = 0.6915 and 1.4288 towards GRB 050820A (z_GRB = 2.615). The properties of the detected DLA galaxy are typical for Lyman-break galaxies (LBGs) at similar redshifts; a young, highly star-forming galaxy that shows evidence for a galactic outflow. This supports thehypothesis that a DLA can be the gaseous halo of an LBG. In addition, we report a redshift coincidence of different objects associated with metal lines in the same field, separated by 130–161 kpc. The high detection rate of three correlated structures on a length scale as short as  ~150 kpc in two pairs of lines of sight is intriguing. The absorbers in each of these are most likely not part of the same gravitationally bound structure. They more likely represent groups of galaxies.