Astronomy & Astrophysics
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 hy-drogen column density, log N(H i) =21.90±0.07, is larger than any (GRB- or QSO-) DLA H column density inferred directly from Lyαin 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/H] =−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 Lyαtrough, a Lyαemission line is detected, which corresponds to a star-formation rate (not corrected for dust extinction) of roughly 1 Myr−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*, 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 volume density to be nH i =102−104 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.
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