2013A&A...557A..18K

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(HI)/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 logf(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 FeII and NiII to d=0.5±0.1kpc. The DLA metallicity ([Zn/H]=-1.15±0.12), visual extinction (A_V≲0.15mag) 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(Hi), 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 Lymanα 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.