IS THE METALLICITY OF THE PROGENITOR OF LONG GAMMA-RAY BURSTS REALLY LOW?

Observations of long gamma-ray bursts (LGRBs) offer a unique opportunity to probe the history of cosmic star formation, although whether LGRBs are biased tracers remains highly debated. Based on an extensive sample of LGRBs compiled by Robertson and Ellis, we analyze various models of star formation rate, combining the possible effect of the cosmic metallicity evolution under the assumption that LGRBs preferentially occur in low-metallicity galaxies. The models of star formation rate tested in this work include empirical fits from observational data as well as a self-consistent model calculated from the hierarchical structure formation scenario. Comparing with the observational data, we find a relatively higher metallicity cut of Z ∼> 0.6 Z☉ for the empirical fits and no metallicity cut for the self-consistent model. These results imply that there is no strong bias toward low metallicity in LGRB host galaxies, in contrast to previous studies suggesting a cut of Z ∼ 0.1-0.3 Z☉, and that the inferred low-metallicity dependencies of LGRBs are strongly related to the specific models of star formation rate. Furthermore, a significant fraction of LGRBs that occur in small halos down to 3 × 108 M☉ can provide an alternative explanation for the difference between the star formation rate and the LGRB rate