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

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(Hi)=cm-2) = 21:95 ± 0:10, prominent H 2 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 (AV 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(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 GRBDLAs, 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. © ESO, 2013.