Probing the Long Gamma-Ray Burst Progenitor by Lyman-alpha Emission of Host Galaxies

TL;DR

This paper explores using Lyman-alpha emission properties of long GRB host galaxies as a tool to infer their metallicity, especially at high redshift where direct measurements are difficult.

Contribution

It proposes a novel method to estimate the metallicity of GRB host galaxies through LAE fraction analysis, incorporating recent theoretical models and effects of metallicity on Ly-alpha emission.

Findings

High LAE fraction in GRB hosts suggests Z < 0.1Z_sun.

Theoretical models can explain observed LAE fractions with metallicity dependence.

LAE statistics can provide insights into GRB progenitor environments.

Abstract

Long gamma-ray bursts (GRBs) have been suggested to occur preferentially in low metallicity environment. We discuss the possibility and theoretical aspects of using Lyman alpha emission properties of long GRB host galaxies as a metallicity indicator of high redshift GRB environments, where direct metallicity measurements are not easy. We propose to use the fraction of Ly-alpha emitters (LAEs) in long GRB host galaxies as a function of UV luminosity, which can be compared with star-formation-rate weighted LAE fraction of Lyman-break galaxies as the standard in the case of no metallicity dependence. There are two important effects of metallicity dependence of long GRB rate to change the LAE fraction of host galaxies. One is the enhancement of intrinsic Ly-alpha equivalent width (EW) by stronger ionizing UV luminosity of low metallicity stellar population, and the other is extinction by interstellar dust to change the observable EW. Based on a latest theoretical model of LAEs that reproduce observations, we argue that the latter is likely to work in the opposite direction to the former, i.e., to decrease LAE fraction if GRBs preferentially occur in low-metallicity environments, because of the clumpy inter-stellar medium effect. The high LAE fraction of GRB host galaxies indicated by observations is quantitatively be explained by the LAE model if GRBs occur when Z < 0.1Z_sun, although this result is still indicative because of the limited statistics and theoretical uncertainties. This result demonstrates that the LAE statistics of GRB hosts may give us useful information in the future.