Revisiting Metallicity of Long Duration Gamma-Ray Burst Host Galaxies: The Role of Chemical Inhomogeneity within Galaxies

TL;DR

This paper models the metallicity distribution of long GRB host galaxies, considering internal chemical inhomogeneity, and finds that high-metallicity hosts can still originate from low-metallicity progenitors, explaining recent observations.

Contribution

It introduces a model for the metallicity distribution of GRB hosts that accounts for internal galaxy metallicity dispersion, reconciling high-metallicity hosts with low-metallicity progenitors.

Findings

GRB hosts may have higher metallicity than progenitors due to internal dispersion.

Over 10% of hosts could have metallicity above 8.8 if dispersion is Milky Way-like.

Possible bimodality in host metallicity PDF could inform progenitor models.

Abstract

We predict the metallicity probability distribution function (PDF) of long gamma-ray burst (GRB) host galaxies at low-redshifts ($z \la 0.3$) when GRBs occur only in low-metallicity environment, assuming empirical formulations of galaxy properties. We discuss contribution of high-metallicity galaxies to the cosmic rate of low-metallicity GRBs, taking internal dispersion of metallicity within each galaxy into account. Assuming GRBs trace low-metallicity star formation $< \Zcrit$: 12+log$_{10}$(O/H) = 8.2, we find that GRB host galaxies may have systematically higher-metallicity than that of GRB progenitors. Furthermore, we expect $\ga$ 10% of the host galaxies to have 12+log$_{10}$(O/H) $> 8.8$, if galaxies have internal dispersion of metallicity comparable to that observed in the Milky Way. Our results show that the low-metallicity scenario of GRB progenitors can be reconciled with the recent discoveries of the high-metallicity host galaxies of GRBs. We also show possible bimodality in the host metallicity PDF that results from the single progenitor model of GRBs. If found in future observation, the bimodality can be a clue to constrain the nature of GRB progenitors.