Molecular hydrogen in the damped Lyman α system towards GRB 120815A at z = 2.36^{⋆,⋆⋆,⋆⋆⋆}

¹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 København Ø, Denmark
e-mail: tom@dark-cosmology.dk
² European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
³ Department of Particle Physics and Astrophysics, Faculty of Physics, Weizmann Institute of Science, 76100 Rehovot, Israel
⁴ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁵ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavik, Iceland
⁶ Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁷ GEPI-Observatoire de Paris, CNRS UMR 8111, Univ. Paris-Diderot, 5 Place Jules Janssen, 92190 Meudon, France
⁸ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
⁹ American River College, Physics and Astronomy Dpt., 4700 College Oak Drive, Sacramento, CA 95841, USA
¹⁰ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹¹ INAF-Osservatorio Astronomico di Roma3, via Frascati 3, 00040 Monteporzio Catone, Italy
¹² ASI-Science Data Centre, via Galileo Galilei, 00044 Frascati, Italy
¹³ Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horska 3a/22, 128 00 Prague 2, Czech Republic
¹⁴ APC, Astroparticules et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cite, 10, rue Alice Domon et Leonie Duquet, 75205 Paris Cedex 13, France
¹⁵ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
¹⁶ Università degli studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
¹⁷ Pontificia Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Casilla 306, Santiago 22, Chile
¹⁸ Millennium Center for Supernova Science  
¹⁹ Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK

Received: 25 April 2013
Accepted: 18 June 2013

Abstract

We present the discovery of molecular hydrogen (H₂), including the presence of vibrationally-excited H₂^* in the optical spectrum of the afterglow of GRB 120815A at z = 2.36 obtained with X-shooter at the VLT. Simultaneous photometric broad-band data from GROND and X-ray observations by Swift/XRT place further constraints on the amount and nature of dust along the sightline. The galactic environment of GRB 120815A is characterized by a strong DLA with log(N(H i)/cm^-2) = 21.95 ± 0.10, prominent H₂ absorption in the Lyman-Werner bands (log (N(H₂)/cm^-2) = 20.54 ± 0.13) and thus a molecular gas fraction log f(H₂) = -1.14 ± 0.15. The distance d between the absorbing neutral gas and GRB 120815A is constrained via photo-excitation modeling of fine-structure and meta-stable transitions of Fe ii and Ni ii to d = 0.5 ± 0.1 kpc. The DLA metallicity ([Zn/H] = -1.15 ± 0.12), visual extinction (A_V ≲ 0.15 mag) and dust depletion ([Zn/Fe] = 1.01 ± 0.10) are intermediate between the values of well-studied, H₂-deficient GRB-DLAs observed at high spectral resolution, and the approximately solar metallicity, highly-obscured and H₂-rich GRB 080607 sightline. With respect to N(H i), metallicity, as well as dust-extinction and depletion, GRB 120815A is fairly representative of the average properties of GRB-DLAs. This demonstrates that molecular hydrogen is present in at least a fraction of the more typical GRB-DLAs, and H₂ and H₂^* are probably more wide-spread among GRB-selected systems than the few examples of previous detections would suggest. Because H₂^* transitions are located redwards of the Lyman α absorption, H₂^* opens a second route for positive searches for molecular absorption also in GRB afterglows at lower redshifts and observed at lower spectral resolution. Further detections of molecular gas in GRB-DLAs would allow statistical studies, and, coupled with host follow-up and sub-mm spectroscopy, provide unprecedented insights into the process and conditions of star-formation at high redshift.