The afterglow and host galaxy of GRB 090205: evidence of a Ly-α emitter at z = 4.65^⋆

¹ INAF-Osservatorio Astronomico di Brera, via Bianchi 46 23807 Merate Italy
e-mail: paolo.davanzo@brera.inaf.it
² ASI Science Data Center, via Galileo Galilei, 00044 Frascati, Italy
³ Università degli Studi dell’Insubria, Dipartimento di Fisica e Matematica, via Valleggio 11, 22100 Como, Italy
⁴ Università degli Studi di Milano-Bicocca, Dipartimento di Fisica, piazza delle Scienze 3, 20126 Milano, Italy
⁵ Department of Astronomy and Astrophysics, Pensylvania State University, 525 Davey Lab, University Park, PA 16802, USA
⁶ Center for Particle Astrophysics, Pensylvania State University, USA
⁷ Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China
⁸ Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain
⁹ The George Washington University, Washington, D.C., 20052, USA
¹⁰ NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹¹ Department of Physics, Pensylvania State University, USA
¹² Department of Physics and Astronomy, University of Nevada, Las Vegas, NV 89154, USA
¹³ INAF-Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone, Roma, Italy
¹⁴ INAF-Osservatorio Astronomico di Capodimonte, salita Moiariello 16, 80131 Napoli, Italy
¹⁵ International Center for Relativistic Astrophysics, piazza della Repubblica 10, 65122 Pescara, Italy

Received: 15 April 2010
Accepted: 28 May 2010

Abstract

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 > 0.27Z_⊙ 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-α emission at the redshift of the burst. GRB 090205 is thus hosted by a typical Lyman-α emitter (LAE) at z = 4.65. This makes the host galaxy of GRB 090205 the farthest GRB host galaxy, spectroscopically confirmed, detected to date.