Molecular hydrogen in the damped Lyman alpha system towards GRB 120815A at z = 2.36

We present the discovery of molecular hydrogen (H<SUB>2</SUB>), including the presence of vibrationally-excited H<SUB>2</SUB><SUP>*</SUP> 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<SUP>-2</SUP>) = 21.95 0.10, prominent H<SUB>2</SUB> absorption in the Lyman-Werner bands (log (N(H<SUB>2</SUB>)/cm<SUP>-2</SUP>) = 20.54 0.13) and thus a molecular gas fraction log f(H<SUB>2</SUB>) = -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<SUB>V</SUB> ≲ 0.15 mag) and dust depletion ([Zn/Fe] = 1.01 0.10) are intermediate between the values of well-studied, H<SUB>2</SUB>-deficient GRB-DLAs observed at high spectral resolution, and the approximately solar metallicity, highly-obscured and H<SUB>2</SUB>-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 H<SUB>2</SUB> and H<SUB>2</SUB><SUP>*</SUP> are probably more wide-spread among GRB-selected systems than the few examples of previous detections would suggest. Because H<SUB>2</SUB><SUP>*</SUP> transitions are located redwards of the Lyman alpha absorption, H<SUB>2</SUB><SUP>*</SUP> 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. Based on observations collected at the European Southern Observatory, Paranal, Chile, Program ID 089.A-0067(B).Appendices are available in electronic form at <A href="http //www.aanda.org">http //www.aanda.org</A>The flux-calibrated average spectrum is only available at the CDS via anonymous ftp to <A href="http //cdsarc.u-strasbg.fr">http //cdsarc.u-strasbg.fr</A> (ftp //130.79.128.5) or via <A href="http //cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/557/A18">http //cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/557/A18</A>