The metallicity of the long GRB hosts and the Fundamental Metallicity Relation of low-mass galaxies

TL;DR

This study examines the metallicity of long GRB host galaxies in relation to the Fundamental Metallicity Relation, revealing that GRB hosts align with the FMR when accounting for their high star formation rates, challenging the idea they prefer low-metallicity environments.

Contribution

It extends the FMR to lower mass galaxies and demonstrates that GRB hosts follow this relation, indicating their metallicity is consistent with their star formation activity.

Findings

GRB hosts are consistent with the extended FMR.

The offset from the mass-metallicity relation is due to higher SFRs.

GRBs do not preferentially occur in low-metallicity galaxies.

Abstract

We investigate the metallicity properties of host galaxies of long Gamma-ray Bursts (GRBs) in the light of the Fundamental Metallicity Relation (FMR), the tight dependence of metallicity on mass and SFR recently discovered for SDSS galaxies with stellar masses above 10^9.2 Msun. As most of the GRB hosts have masses below this limit, the FMR can only be used after an extension towards lower masses. At this aim, we study the FMR for galaxies with masses down to ~10^8.3 Msun, finding that the FMR does extend smoothly at lower masses, albeit with a much larger scatter. We then compare the resulting FMR with the metallicity properties of 18 host galaxies of long GRBs. While the GRB hosts show a systematic offset with respect to the mass-metallicity relation, they are fully consistent with the FMR. This shows that the difference with the mass-metallicity relation is due to higher than average SFRs, and that GRBs with optical afterglows do not preferentially select low-metallicity hosts among the star-forming galaxies. The apparent low metallicity is therefore a consequence of the occurrence of long GRB in low mass, actively star-forming galaxies, known to dominate the current cosmic SFR.