The afterglow and host galaxy of GRB 090205 evidence of a Ly-alpha emitter at z = 4.65

Context. Gamma-ray bursts (GRBs) have proven to be detectable to distances much larger than any other astrophysical object, providing the most effective way, complementing ordinary surveys of studying the high redshift universe. Aims. We present the results of an observational campaign devoted to the study of the high-z GRB 090205. Methods. We carried out optical/NIR spectroscopy and imaging of GRB 090205 with the ESO-VLT starting from hours after the event to several days later to detect the host galaxy. We compared the results obtained from our optical/NIR observations with the available Swift high-energy data of this burst. Results. Our observational campaign led to the detection of the optical afterglow and host galaxy of GRB 090205 and to the first measure of its redshift, z = 4.65. As in other high-z GRBs, GRB 090205 has a short duration in the rest frame with T_(90,rf) = 1.6 s, which suggests that it might belong to the short GRB class. The X-ray afterglow of GRB 090205 has a complex and interesting behavior with a possible rebrightening at 500-1000 s from the trigger time and late flaring activity. Photometric observations of the GRB 090205 host galaxy imply that it is a starburst galaxy with a stellar population younger than ~150 Myr. Moreover, the metallicity of Z > 0.27 Z⊙ derived from the GRB afterglow spectrum is among the highest derived from GRB afterglow measurement at high-z, suggesting that the burst occurred in a rather enriched environment. Finally, a detailed analysis of the afterglow spectrum shows the existence of a line corresponding to Lyman-alpha emission at the redshift of the burst. GRB 090205 is thus hosted by a typical Lyman-alpha emitter (LAE) at z = 4.65. This makes the host galaxy of GRB 090205 the farthest GRB host galaxy, spectroscopically confirmed, detected to date.