The metallicity of the long GRB hosts and the fundamental metallicity relation of low-mass galaxies

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 star formation rate (SFR) recently discovered for Sloan Digital Sky Survey galaxies with stellar masses above 10(9.2) M-circle dot. As most of the GRB hosts have masses below this limit, the FMR can only be used after an extension towards lower masses. With this aim, we study the FMR for galaxies with masses down to similar to 10(8.3) M-circle dot, 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 themass-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 a long GRB in low-mass, actively star-forming galaxies, known to dominate the current cosmic SFR.