On the Lyα emission from gamma-ray burst host galaxies:: Evidence for low metallicities

We report on the results of a search for Lyalpha emission from the host galaxy of the z = 2.140 GRB 011211 and other galaxies in its surrounding field. We detect Lyalpha emission from the host as well as from six other galaxies in the field. The restframe equivalent width of the Lyalpha line from the GRB 011211 host is about 21 Angstrom. This is the fifth detection of Lyalpha emission out of five possible detections from GRB host galaxies, strongly indicating that GRB hosts, at least at high redshifts, are Lyalpha emitters. This is intriguing as only similar to25% of the Lyman-Break selected galaxies at similar redshifts have Lyalpha emission lines with restframe equivalent width larger than 20 Angstrom. Possible explanations are i) a preference for GRB progenitors to be metal-poor as expected in the collapsar model, ii) an optical afterglow selection bias against dusty hosts, and iii) a higher fraction of Lyalpha emitters at the faint end of the luminosity function for high-z galaxies. Of these, the current evidence seems to favour i).