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Theoretically long gamma-ray bursts (GRBs) are expected to happen inlow-metallicity environments, because in a single massive star scenario, lowiron abundance prevents loss of angular momentum through stellar wind,resulting in ultra-relativistic jets and the burst. In this sense, not just asimple metallicity measurement but also low iron abundance ([Fe/H]<-1.0) isessentially important. Observationally, however, oxygen abundance has beenmeasured more often due to stronger emission. In terms of oxygen abundance,some GRBs have been reported to be hosted by high-metallicity star-forminggalaxies, in tension with theoretical predictions. Here we compare iron andoxygen abundances for the first time for GRB host galaxies (GRB 980425 and080517) based on the emission-line diagnostics. The estimated total ironabundances, including iron in both gas and dust, are well below the solarvalue. The total iron abundances can be explained by the typical value oftheoretical predictions ([Fe/H]<-1.0), despite high oxygen abundance in one ofthem. According to our iron abundance measurements, the single massive starscenario still survives even if the oxygen abundance of the host is very high,such as the solar value. Relying only on oxygen abundance could mislead us onthe origin of the GRBs. The measured oxygen-to-iron ratios, [O/Fe], can becomparable to the highest values among the iron-measured galaxies in the SloanDigital Sky Survey. Possible theoretical explanations of such high [O/Fe]include the young age of the hosts, top-heavy initial mass function, andfallback mechanism of the iron element in supernova explosions.

Why Are Some Gamma-Ray Bursts Hosted by Oxygen-rich Galaxies?