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

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. GreinerO.~E. Hartoog, B. Milvang-Jensen, M. Nardini, S. Piranomonte, A. Rossi, R. Sánchez-Ramirez, P. Schady, S. Schulze, V. Sudilovsky, N.~R. Tanvir, G. Tagliaferri, D.~J. Watson, K. Wiersema, R.~A.~M.~J. Wijers, D. Xu

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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 (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(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.
Original languageEnglish
Article numberA18
Pages (from-to)1-21
Number of pages21
JournalAstronomy & Astrophysics
Early online date14 Aug 2013
Publication statusPublished - Sep 2013


  • galaxies
  • gamma-ray burst
  • high-redshift
  • molecules
  • dust
  • extinction

Cite this

Krühler, T., Ledoux, C., Fynbo, J. P. U., Vreeswijk, P. M., Schmidl, S., Malesani, D., Christensen, L., De Cia, A., Hjorth, J., Jakobsson, P., Kann, D. A., Kaper, L., Vergani, S. D., Afonso, P. M. J., Covino, S., de Ugarte Postigo, A., D'Elia, V., Filgas, R., Goldoni, P., ... Xu, D. (2013). Molecular hydrogen in the damped Lyman alpha system towards GRB 120815A at z = 2.36. Astronomy & Astrophysics, 557, 1-21. [A18].