Abstract
Aims. We have gathered optical spectra of 8 long-duration GRB host galaxies selected from the archival data of VLT/FORS2. We investigated whether or not Wolf-Rayet (WR) stars can be detected in these GRB host galaxies. We also tried to estimate the physical properties of GRB host galaxies, such as metallicity. Methods. We identified the WR features in these spectra by fitting the WR bumps and WR emission lines in blue and red bumps. We also identified the subtypes of the WR stars, and estimated the numbers of stars in each subtype, then calculated the WR/O star ratios. The (O/H) abundances of GRB hosts were estimated from both the electron temperature (Te) and the metallicity-sensitive strong-line ratio (R23), for which we have broken the R23 degeneracy. We compared the environments of long-duration GRB host galaxies with those of other galaxies in terms of their luminosity (stellar mass)-metallicity relations (LZ, MZ). Results. We detected the presence of WR stars in 5 GRB host galaxies having spectra with relatively high signal-to-noise ratios (S/N). In the comparison of LZ, MZ relations, it shows that GRB hosts have lower metallicities than other samples with comparable luminosity and stellar mass. The presence of WR stars and the observed high WR/O star ratio, together with low metallicity, support the core-collapsar model and implie the first stage of star formation in the hosted regions of GRBs.
Highlights
Gamma-ray bursts (GRBs), the most energetic events in the Universe, were discovered accidentally in the 1960s (Klebesadel et al 1973)
Short GRBs are believed to originate from the merger of compact binaries such as double neutron star binaries (NS-NS) and black hole-neutron star binaries (BH-NS)
We identify the WR features of these 5 GRB hosts based on the appearance of the blue bump and WR emission lines by fitting them
Summary
Gamma-ray bursts (GRBs), the most energetic events in the Universe, were discovered accidentally in the 1960s (Klebesadel et al 1973). Evidence from photometric and spectroscopic observations shows that the host galaxies of long-duration GRBs are mostly faint, blue, low-mass, star-forming galaxies with low metallicities (Sokolov et al 2001; Le Floc’h et al 2003; Fynbo et al 2003; Courty et al 2004; Prochaska et al 2004; Christensen et al 2004; Chary et al 2002; Gorosabel et al 2005; Fynbo et al 2006; Wiersema et al 2007; Kewley et al 2007; Levesque et al 2009; Savaglio et al 2009, hereafter Savaglio). We try to detect the WR features in these GRB hosts study the physical properties of GRB host galaxies, such as their metallicities and luminosity (stellar mass)–metallicity relations (L − Z, M∗ − Z).
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