The metallicity properties of long-GRB hosts

The recently-discovered Fundamental Metallicity Relation (FMR), which is the tight dependence of metallicity on both mass and SFR, proves to be a very useful tool to study the metallicity properties of various classes of galaxies. We have used the FMR to study the galaxies hosting long-GRBs. While the GRB hosts have lower metallicities than typical galaxies of the same mass, i.e., they are below 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.

💡 Research Summary

This paper presents a significant analysis of long-duration Gamma-Ray Burst (GRB) host galaxies through the lens of the Fundamental Metallicity Relation (FMR), a tight correlation between a galaxy’s gas-phase metallicity, its stellar mass, and its star formation rate (SFR).

The study begins by establishing the FMR using a large sample of local galaxies from the SDSS survey. It demonstrates that at a fixed stellar mass, galaxies with higher SFRs exhibit lower metallicities. When plotted in the three-dimensional space of mass, SFR, and metallicity, local galaxies define a remarkably tight surface, the FMR, with a scatter of only about 0.05 dex. Intriguingly, the authors find that star-forming galaxies at redshifts up to z ~ 2.5 also lie on this same surface, showing no significant evolution of the FMR itself. The observed evolution of the simpler mass-metallicity relation with redshift is explained as a selection effect: high-redshift galaxies have systematically higher SFRs, placing them in a different region of the non-evolving FMR.

The core of the paper applies this powerful framework to long-GRB host galaxies. A leading theoretical model for long GRBs suggests they originate from the collapse of massive, low-metallicity stars. Observations have consistently shown that GRB host galaxies tend to have lower metallicities than typical galaxies of the same mass, seemingly supporting this metallicity bias.

To test this, the authors compile a sample of GRB hosts at z < 1 with consistently measured stellar masses, SFRs (from H-alpha), and metallicities. When compared to the local mass-metallicity relation, these hosts confirm the known offset towards lower metallicities. However, when their position is evaluated against the 3D FMR, a different story emerges: the GRB hosts fall perfectly on the FMR defined by normal star-forming galaxies. Their apparently low metallicity is entirely accounted for by their combination of relatively low mass and high SFR.

This result carries profound implications. It suggests that long GRBs with optical afterglows (which constitute the study’s sample) do not preferentially select galaxies with anomalously low metallicity. Instead, they trace normal, actively star-forming galaxies. The low metallicity of their hosts is not a special prerequisite but a natural consequence of the FMR for galaxies with high specific star formation rates. The paper notes that “dark” GRBs, likely hosted by dustier and potentially more metal-rich galaxies, might populate a different part of the FMR.

Finally, the authors highlight a major application: because GRB hosts appear to be representative star-forming galaxies, large future samples of GRB hosts, potentially extending to very high redshifts (z > 6), can be used as powerful probes to study the evolution of the FMR itself throughout cosmic history, in epochs otherwise difficult to access.