T. Krühler, C. Ledoux, J. P. U. Fynbo, P. M. Vreeswijk, S. Schmidl, D. Malesani, L. Christensen, A. De Cia, J. Hjorth, P. Jakobsson, D. A. Kann, L. Kaper, S. D. Vergani, P. M. J. Afonso, S. Covino, A. de Ugarte Postigo, V. D'Elia, R. Filgas, P. Goldoni, J. Greiner, O. E. Hartoog, B. Milvang-Jensen, M. Nardini, S. Piranomonte, A. Rossi, R. Sánchez-Ramírez, P. Schady, S. Schulze, V. Sudilovsky, N. R. Tanvir, G. Tagliaferri, D. J. Watson, K. Wiersema, R. A. M. J. Wijers, D. Xu
[Abridged] Molecular hydrogen H_2 is a key requirement for star-formation, frequently observed along sightlines in the Galaxy and to the Magellanic Clouds, but notoriously hard to detect directly beyond z ~ 0. In the DLAs associated with long gamma-ray bursts (GRBs), which are tightly linked to vigorous star-formation, H_2 has remained largely elusive, and has been unequivocally detected only towards GRB 080607 and possibly towards GRB 060206. Here, we present the discovery of H_2-rich gas, including the presence of vibrationally-excited H_2^* in the optical spectrum of the afterglow of GRB 120815A at z=2.36 obtained with X-shooter at the VLT. The galactic environment of GRB 120815A is characterized by a strong DLA with log(N(H)/cm^-2) = 21.95 +/- 0.10, prominent H_2 absorption in the Lyman-Werner bands (log(N(H_2)/cm^-2) = 20.53 +/- 0.04) and thus a molecular gas fraction log f(H_2)=-1.14 +/- 0.10. 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 FeII and NiII to d = 0.5 +/- 0.1 kpc. The DLA metallicity ([Zn/H] = -1.15 +/- 0.11), visual extinction (A_V < 0.15 mag) and dust depletion ([Zn/Fe] = 1.01 +/- 0.09) are intermediate between the values of well-studied, H_2-deficient GRB-DLAs observed at high spectral resolution, and the approximately solar metallicity, highly-obscured and H_2-rich GRB 080607 sightline. With respect to N(H), 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 H_2 and H_2^* are probably more wide-spread among GRB-selected systems than the few examples of previous detections would suggest.
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http://arxiv.org/abs/1304.7003
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