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

摘要

We investigate the metallicity properties of host galaxies of long Gamma-rayBursts (GRBs) in the light of the Fundamental Metallicity Relation (FMR), thetight dependence of metallicity on mass and SFR recently discovered for SDSSgalaxies with stellar masses above 10^9.2 Msun. As most of the GRB hosts havemasses below this limit, the FMR can only be used after an extension towardslower 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, albeitwith a much larger scatter. We then compare the resulting FMR with themetallicity properties of 18 host galaxies of long GRBs. While the GRB hostsshow a systematic offset with respect to the mass-metallicity relation, theyare fully consistent with the FMR. This shows that the difference with themass-metallicity relation is due to higher than average SFRs, and that GRBswith optical afterglows do not preferentially select low-metallicity hostsamong the star-forming galaxies. The apparent low metallicity is therefore aconsequence of the occurrence of long GRB in low mass, actively star-forminggalaxies, known to dominate the current cosmic SFR.