The host of GRB 030323 at z = 3.372: A very high column density DLA system with a low metallicity

Abstract

We present photometry and spectroscopy of the afterglow of GRB 030323. VLT spectra of the afterglow show damped Lyα (DLA) absorption and low- and high-ionization lines at a redshift z = 3.3718 ± 0.0005. The inferred neutral hydrogen column density, log N(H I) = 21.90 ± 0.07, is larger than any (GRB- or QSO-) DLA H I column density inferred directly from Lyα in absorption. From the afterglow photometry, we derive a conservative upper limit to the host-galaxy extinction: Av < 0.5 mag. The iron abundance is [Fe/H] = -1.47 ± 0.11, while the metallicity of the gas as measured from sulphur is [S/HJ = -1.26 ± 0.20. We derive an upper limit on the H2 molecular fraction of 2N(H2)/(2N(H2) + N(H I)) ≲ 10-6. In the Lyα trough, a Lyα emission line is detected, which corresponds to a star-formation rate (not corrected for dust extinction) of roughly 1 Modot; yr-1. All these results are consistent with the host galaxy of GRB 030323 consisting of a low metallicity gas with a low dust content. We detect fine-structure lines of silicon, Si II*, which have never been clearly detected in QSO-DLAs; this suggests that these lines are produced in the vicinity of the GRB explosion site. Under the assumption that these fine-structure levels are populated by particle collisions, we estimate the H I volume density to be nH I = 102-102 cm-3. HST/ACS imaging 4 months after the burst shows an extended AB(F606W) = 28.0 ± 0.3 mag object at a distance of 0″.14 (1 kpc) from the early afterglow location, which presumably is the host galaxy of GRB 030323.