Metallicity Measurements of Gamma-Ray Burst and Supernova Explosion Sites: Lessons from HII regions in M31

TL;DR

This study investigates how small-scale metallicity variations within galaxies influence the observational estimates of explosion site metallicities for supernovae and gamma-ray bursts, highlighting the impact of spatial resolution on these measurements.

Contribution

It demonstrates that spatial resolution significantly affects metallicity estimates, showing that high apparent metallicities at coarse resolutions may not reflect local environments.

Findings

At resolutions below 0.5 kpc, metallicity estimates closely match target regions.

At resolutions above 1.0 kpc, metallicity estimates reflect the average of surrounding regions.

High apparent metallicities do not necessarily indicate high local metallicity environments.

Abstract

We examine how the small-scale ($<$\,kpc) variation of metallicity within a galaxy, which is found in nearby galaxies, affect the observational estimates of metallicity in the explosion sites of transient events such as core-collapse supernovae (CC SNe) and gamma-ray bursts (GRBs). Assuming the same luminosity, metallicity, and spatial distributions of \ion{H}{ii}\ regions (hereafter HIIRs) as observed in M31, we compute the apparent metallicities that we would obtain when the spectrum of a target region is blended with those of surrounding HIIRs within the length scale of typical spatial resolution. When the spatial resolution of spectroscopy is $\lesssim$ 0.5\,kpc, which is typical for the existing studies of CC SN sites, we find that the apparent metallicities reflect the metallicities of target regions, but with significant systematic uncertainties in some cases. When the spatial resolution is $\gtrsim$ 1.0 kpc, regardless of the target regions (which has a wide range of metallicity that spans $\sim$ 0.6 dex for the M31 HIIRs), we always obtain the apparent metallicities similar to the average metallicity of the M31 HIIRs. Given that the apparent metallicities measured with $\gtrsim$\,kpc scale resolution do not necessarily reflect the immediate environment of the stellar explosions, the current observational estimates of high metallicities for some of the long GRB host galaxies do {\em not} rule out the hypothesis that the long GRBs are exclusively born in a low-metallicity environment.