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A&A 419, 927-940 (2004)
DOI: 10.1051/0004-6361:20040086

The host of GRB 030323 at $\mathsf{\textit{z}=3.372}$: A very high column density DLA system with a low metallicity

P. M. Vreeswijk1, S. L. Ellison1, 2, C. Ledoux1, R. A. M. J. Wijers3, J. P. U. Fynbo4, 5, P. Møller6, A. Henden7, J. Hjorth5, G. Masi8, E. Rol3, B. L. Jensen5, N. Tanvir9, A. Levan10, J. M. Castro Cerón11, J. Gorosabel11, 12, A. J. Castro-Tirado12, A. S. Fruchter11, C. Kouveliotou13, I. Burud11, J. Rhoads11, N. Masetti14, E. Palazzi14, E. Pian14, 15, H. Pedersen5, L. Kaper3, A. Gilmore16, P. Kilmartin16, J. V. Buckle17, M. S. Seigar17, D. H. Hartmann18, K. Lindsay18 and E. P. J. van den Heuvel3

1  European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
2  P. Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
3  Astronomical Institute "Anton Pannekoek", University of Amsterdam & Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
4  Department of Physics and Astronomy, Århus University, Ny Munkegade, 8000 Århus C, Denmark
5  Niels Bohr Institute, Astronomical Observatory, Copenhagen University, Juliane Mariesvej 30, 2100 København Ø, Denmark
6  European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
7  Universities Space Research Association / US Naval Observatory Flagstaff Station, PO Box 1149, Flagstaff AZ 86002-1149, USA
8  Physics Department, University of Rome "Tor Vergata", Via della Ricerca Scientifica 1, 00133 Rome, Italy
9  Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK
10  Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
11  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218-2463, USA
12  Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado de Correos, 3004, 18080 Granada, Spain
13  NSSTC, SD-50, 320 Sparkman Dr., Huntsville, AL 35805, USA
14  Istituto di Astrofisica Spaziale e Fisica Cosmica - Sezione di Bologna, CNR, via Gobetti 101, 40129 Bologna, Italy
15  INAF - Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
16  Department of Physics and Astronomy, University of Canterbury, Christchurch, New Zealand
17  Joint Astronomy Centre, 660 N, A'ohoku Place, Hilo, Hawaii, HI 96720, USA
18  Department of Physics & Astronomy, Clemson University, Clemson, SC 29634-0978, USA

(Received 17 July 2003 / Accepted 27 February 2004 )

Abstract
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, $\log N({\ion})=21.90\pm0.07$, is larger than any (GRB- or QSO-) DLA $\ion$ 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 $2N({\rm H_2)}/(2N{\rm (H_2)}+N({\ion}))\la10^$. 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, $\ion{ii}$*, 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 $\ion$ volume density to be $n_{\ion}=10^2{-}10^4$ 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.


Key words: gamma rays: bursts -- galaxies: distances and redshifts -- galaxies: quasars: absorption lines -- ISM: dust, extinction

Offprint request: P. M. Vreeswijk, pvreeswi@eso.org

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