H I column densities of z > 2 Swift gamma-ray bursts

A&A 460, L13-L17 (2006)
DOI: 10.1051/0004-6361:20066405

Letter

H I column densities of z > 2 Swift gamma-ray bursts

P. Jakobsson^{1, 2, 3}, J. P. U. Fynbo², C. Ledoux⁴, P. Vreeswijk^{4, 5}, D. A. Kann⁶, J. Hjorth², R. S. Priddey¹, N. R. Tanvir⁷, D. Reichart⁸, J. Gorosabel⁹, S. Klose⁶, D. Watson², J. Sollerman², A. S. Fruchter¹⁰, A. de Ugarte Postigo⁹, K. Wiersema¹¹, G. Björnsson³, R. Chapman¹, C. C. Thöne², K. Pedersen², and B. L. Jensen²

¹ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK
² Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
³ Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland
⁴ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
⁵ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁶ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁷ Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
⁸ Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Campus Box 3255, Chapel Hill, NC 27599, USA
⁹ Instituto de Astrofísica de Andalucía (CSIC), Apartado de Correos 3004, 18080 Granada, Spain
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

(Received 15 September 2006 / Accepted 10 October 2006)

Abstract
Context.Before the launch of the Swift satellite, the majority of the gamma-ray burst (GRB) afterglows for which Ly $\alpha$ was redshifted into the observable spectrum showed evidence for a damped Ly $\alpha$ absorber. This small sample indicated that GRBs explode either in galaxies, or regions within them, having high neutral hydrogen column densities.
Aims.To increase the spectroscopic sample of GRBs with z>2 and hence establish the $N(\ion{H}{i})$ distribution along GRB lines-of-sight.
Methods.We have obtained six z > 2 GRB afterglow spectra and fitted the Ly $\alpha$ absorption line in each case to determine $N(\ion{H}{i})$ . This has been complemented with 12 other Swift $N(\ion{H}{i})$ values from the literature.
Results.We show that the peak of the GRB $N(\ion{H}{i})$ distribution is qualitatively consistent with a model where GRBs originate in Galactic-like molecular clouds. However, a systematic difference, in particular an excess of low column-density systems compared to the predictions, indicates that selection effects and conditions within the cloud (e.g. strong ionization) influence the observed $N(\ion{H}{i})$ range. We also report the discovery of Ly $\alpha$ emission from the GRB 060714 host, corresponding to a star-formation rate of approximately 0.8 $M_{\odot}\,{\rm yr}^{-1}$ . Finally, we present accurate redshifts of the six bursts: $z = 3.240 \pm 0.001$ (GRB 050319), $z = 2.198 \pm 0.002$ (GRB 050922C), $z = 3.221 \pm 0.001$ (GRB 060526), $z = 3.425 \pm 0.002$ (GRB 060707), $z = 2.711 \pm 0.001$ (GRB 060714) and $z = 3.686 \pm 0.002$ (GRB 060906).

Key words: gamma rays: bursts -- galaxies: high-redshift -- galaxies: abundances -- dust, extinction