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Issue A&A
Volume 388, Number 2, June III 2002
Page(s) 425 - 438
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20020427



A&A 388, 425-438 (2002)
DOI: 10.1051/0004-6361:20020427

Deep Ly $\alpha$ imaging of two ${\vec{z}}$ = 2.04 GRB host galaxy fields

J. P. U. Fynbo1, P. Møller1, B. Thomsen2, J. Hjorth3, J. Gorosabel4, 5, 6, M. I. Andersen7, M. P. Egholm8, 2, 1, S. Holland9, B. L. Jensen3, H. Pedersen3 and M. Weidinger2, 1

1  European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748, Garching by München, Germany
2  Institute for Physics and Astronomy University of Århus, Ny Munkegade, 8000 Århus C, Denmark
3  Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
4  Danish Space Research Institute, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
5  Laboratorio de Astrofísica Espacial y Física Fundamental (LAEFF-INTA), PO Box 50727, 28080 Madrid, Spain
6  Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 03004, 18080 Granada, Spain
7  Division of Astronomy, PO Box 3000, 90014 University of Oulu, Finland
8  Nordic Optical Telescope, Apartado Postal 474, 38700 Santa Cruz de La Palma, Canary Islands, Spain
9  Department of Physics, University of Notre Dame, Notre Dame, IN 46556-5670, USA

(Received 4 December 2001 / Accepted 19 March 2002)

Abstract
We report on the results of deep narrow-band Ly $\alpha$ and broad-band U and I imaging of the fields of two Gamma-Ray bursts at redshift z=2.04 (GRB 000301C and GRB 000926). We find that the host galaxy of GRB 000926 is an extended (more than 2 arcsec), strong Ly $\alpha$ emitter with a rest-frame equivalent width of 71 +20-15 Å. The galaxy consists of two main components and several fainter knots. GRB 000926 occurred in the western component, whereas most of the Ly $\alpha$ luminosity (about 65%) originates in the eastern component. Using archival HST images of the host galaxy we measure the spectral slopes ( $f_{\lambda} \propto \lambda^{\beta}$) of the two components to $\beta = -2.4\pm0.3$ (east) and $-1.4\pm0.2$ (west). This implies that both components contain at most small amounts of dust, consistent with the observed strong Ly $\alpha$ emission. The western component has a slightly redder V-I colour than the eastern component, suggesting the presence of at least some dust. We do not detect the host galaxy of GRB 000301C in neither Ly $\alpha$ emission nor in U and I broad-band images. The strongest limit comes from combining the narrow and U-band imaging where we infer a limit of U(AB)>27.7 (2 $\sigma$ limit per arcsec 2). The upper limits on the Ly $\alpha$ flux implies a Ly $\alpha$ equivalent width upper limit of ~150 Å. We find eleven and eight other galaxies with excess emission in the narrow filter in the fields of GRB 000301C and GRB 000926 respectively. These galaxies are candidate Ly $\alpha$ emitting galaxies in the environment of the host galaxies. Based on these detections we conclude that GRB 000926 occurred in one of the strongest centres of star formation within several Mpc, whereas GRB 000301C occurred in an intrinsically very faint galaxy far from being the strongest centre of star formation in its galactic environment. Under the hypothesis that GRBs trace star formation, the wide range of GRB host galaxy luminosities implies a very steep faint end slope of the high redshift galaxy luminosity function.


Key words: cosmology: observations -- gamma rays: bursts

Offprint request: J. P. U. Fynbo, jfynbo@eso.org

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© ESO 2002


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