[Paper Review] A Surprising Lack of Metallicity Evolution with Redshift in the Long Gamma-Ray Burst Host Galaxy Population

J. F. Graham, P. Schady+1 more|arXiv (Cornell University)|Apr 4, 2019

Gamma-ray bursts and supernovae4 citations

TL;DR

This study analyzes the metallicity evolution of long gamma-ray burst (LGRB) host galaxies using a sample of over 100 galaxies across redshifts up to z=2.5. Contrary to expectations, it finds no significant evolution in the metallicity distribution—particularly, the fraction of hosts with high metallicity (12+log(O/H) ≥ 8.4) remains constant—challenging the assumption that LGRB hosts become more metal-poor at higher redshifts.

ABSTRACT

The number of long-duration Gamma Ray Burst (LGRB) host galaxies with measured metallicities and host masses has now grown to over one hundred, allowing us to investigate how the distributions of both these properties change with redshift. Using the combined host galaxy metallicity sample from Graham & Fruchter (2013) and Krühler et al. (2015), we find a surprising lack of evolution in the LGRB metallicity distribution across different redshifts. In particular, the fraction of LGRB hosts with relatively high metallicity (12+log(O/H) $\geq$ 8.4) remains essentially constant out to z = 2.5. This result is at odds with the evolution in the mass-metallicity relation of typical galaxies, which become progressively more metal poor with increasing redshift. A similar result is found when converting the LGRB host galaxy mass distribution taken from the SHOALS (Swift GRB Host Galaxy Legacy Survey) sample to a corresponding metallicity distribution by applying a redshift-dependent mass-metallicity relation. The SHOALS sample is compiled using an unbiased selection function implying that the observed lack of evolution in the host galaxy high metallicity distribution is not caused by selection effects. However, the LGRB host galaxy metallicities estimated from the stellar mass are typically a quarter dex higher at all redshifts than the metallicity we measure spectroscopically. This implies that using mass-metallicity relationships to estimate host metallicities will thus produce a substantial systematic bias.

Motivation & Objective

• To investigate whether the metallicity distribution of long gamma-ray burst (LGRB) host galaxies evolves with cosmic redshift.
• To test whether selection effects or biases in host galaxy sampling could explain apparent metallicity trends.
• To compare spectroscopically measured metallicities with those estimated from stellar mass using the mass-metallicity relation (MZR).
• To assess the reliability of mass-based metallicity estimates for LGRB hosts in cosmological studies.

Proposed method

• Combined spectroscopic metallicities from Graham & Fruchter (2013) and Krühler et al. (2015) for LGRB hosts with redshifts up to z=2.5.
• Applied the KK04 and CCM+17 metallicity diagnostics to ensure consistency and expand sample coverage, especially for objects with incomplete line fluxes.
• Used the Kolmogorov-Smirnov (K-S) test to compare cumulative metallicity distributions across redshift bins (Δz=0.5) to detect evolution.
• Converted stellar mass distributions from the SHOALS survey into metallicity estimates using a redshift-dependent mass-metallicity relation from Zahid et al. (2013).
• Performed statistical comparisons between measured metallicities and mass-derived estimates, quantifying systematic offsets.
• Analyzed subsamples including only full-line-coverage objects and expanded samples with partial-line diagnostics to assess methodological sensitivity.

Experimental results

Research questions

• RQ1 Does the metallicity distribution of LGRB host galaxies evolve with increasing redshift, as expected from cosmic chemical evolution?
• RQ2 Are the observed high-metallicity LGRB hosts (12+log(O/H) ≥ 8.4) consistent with a constant fraction across cosmic time, or is this a selection bias?
• RQ3 How do metallicity estimates derived from stellar mass compare to spectroscopically measured metallicities in LGRB hosts?
• RQ4 To what extent do mass-metallicity relations introduce systematic biases when applied to LGRB host galaxies?

Key findings

• The fraction of LGRB hosts with high metallicity (12+log(O/H) ≥ 8.4) remains constant at approximately 20% from z=0 to z=2.5, showing no significant evolution.
• The K-S test yields p-values >0.3 for all redshift bin comparisons, indicating that the metallicity distributions are statistically indistinguishable across redshifts.
• Mass-based metallicity estimates are systematically higher than spectroscopic measurements by a mean offset of 0.23 dex, with a median offset of 0.30 dex.
• The standard deviation of the offset between estimated and measured metallicities is 0.46 dex, indicating high scatter and poor reliability for correction.
• In raw (non-logarithmic) terms, the median ratio of measured to estimated metallicities is 0.496, confirming that mass-based estimates overpredict true metallicities.
• The lack of redshift evolution persists even when using the CCM+17 diagnostic, which includes additional objects with incomplete line coverage, confirming robustness to methodological variations.