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

We present photometry and spectroscopy of the afterglow of GRB 030323. VLT spectra of the afterglow show damped Ly $\alpha$ (DLA) absorption and low- and high-ionization lines at a redshift $z=3.3718\pm0.0005$. The inferred neutral hydrogen column density, $\log N({\ion{H}{i}})=21.90\pm0.07$, is larger than any (GRB- or QSO-) DLA $\ion{H}{i}$ column density inferred directly from Ly $\alpha$ in absorption. From the afterglow photometry, we derive a conservative upper limit to the host-galaxy extinction: $A_{\rm V}<0.5$ mag. The iron abundance is $\rm [Fe/H]=-1.47\pm0.11$, while the metallicity of the gas as measured from sulphur is $\rm [S/H]=-1.26\pm0.20$. We derive an upper limit on the H 2 molecular fraction of $2N({\rm H_2)}/(2N{\rm (H_2)}+N({\ion{H}{i}}))\la10^{-6}$. In the Ly $\alpha$ trough, a Ly $\alpha$ emission line is detected, which corresponds to a star-formation rate (not corrected for dust extinction) of roughly 1 M $_{\odot}$ 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, $\ion{Si}{ii}$*, 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 $\ion{H}{i}$ volume density to be $n_{\ion{H}{i}}=10^2{-}10^4$ cm -3. HST/ACS imaging 4 months after the burst shows an extended ${\it AB}(F606W)=28.0\pm0.3$ mag object at a distance of 014 (1 kpc) from the early afterglow location, which presumably is the host galaxy of GRB 030323.