Metallicity measurements of GRB explosion sites: Lessons from H II regions in M31

Abstract

We examine how the small-scale variation of metallicity within a galaxy, which is found in nearby galaxies, affect the observational estimates of metallicity in the explosion sites of gamma-ray bursts (GRBs). Assuming the same luminosity, metallicity, and spatial distributions of H II regions as observed in M31, we compute the apparent metallicities that we would obtain when the spectrum of a target region is blended with those of surrounding H II regions within the length scale of typical spatial resolution. When the spatial resolution is ≳ a few kpc, we always obtain the apparent metallicity of 12+log(O/H) ∼ 8.8 (average metallicity of H II regions in M31) regardless of the actual metallicities of the target regions (which has a wide range of 12+log(O/H) = 8.1-9.3 for the M31 H II regions). Our results suggest that current observational estimates of high-metallicity for some long-GRB environments do not rule out the hypothesis that long GRBs are exclusively born in low-metallicity environment.