The Wolf-Rayet features and mass–metallicity relation of long-duration gamma-ray burst host galaxies

¹ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, the Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China e-mail: hxh@nao.cas.cn
² GEPI, Observatoire de Paris-Meudon, Meudon 92195, France e-mail: francois.hammer@obspm.fr
³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
⁴ Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR China

Received: 12 May 2009
Accepted: 6 January 2010

Abstract

Aims. We gather optical spectra of 8 long-duration GRB host galaxies selected from the archival data of VLT/FORS2. We investigate whether or not Wolf-Rayet (WR) stars can be detected in these GRB host galaxies. We also estimate the physical properties of GRB host galaxies, such as metallicity.

Methods. We identify the WR features in these spectra by fitting the WR bumps and WR emission lines in blue and red bumps. We identify the subtypes of the WR stars, estimate the numbers of stars in each subtype, and calculate the WR/O star ratios. The (O/H) abundances of GRB hosts are inferred from both the electron temperature (T_e) and the metallicity-sensitive strong-line ratio (R₂₃), for which we break the R₂₃ degeneracy. We compare the environments of long-duration GRB host galaxies with those of other galaxies in terms of their luminosity (stellar mass)–metallicity relations (L–Z, –Z).

Results. We detect WR stars in 5 GRB host galaxies with spectra of relatively high signal-to-noise ratios (S/N). In the comparison of L–Z, –Z relations, we show that GRB hosts have lower metallicities than other samples of comparable luminosity and stellar mass. The presence of WR stars and the observed high WR/O star ratio, together with the low metallicity, support the core-collapsar” model and imply that we are witnessing the first stage of star formation in the host regions of GRBs.