Abstract
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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