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

We report on the results of a search for Lyα emission from the host galaxy of the z = 2.140 GRB 011211 and other galaxies in its surrounding field. We detect Lyα emission from the host as well as from six other galaxies in the field. The restframe equivalent width of the Lyα line from the GRB 011211 host is about 21 Å. This is the fifth detection of Lyα emission out of five possible detections from GRB host galaxies, strongly indicating that GRB hosts, at least at high redshifts, are Lyα emitters. This is intriguing as only ∼25% of the Lyman-Break selected galaxies at similar redshifts have Lyα emission lines with restframe equivalent width larger than 20 Å. 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 Lyα emitters at the faint end of the luminosity function for high-z galaxies. Of these, the current evidence seems to favour i).