[Paper Review] A Surprising Lack of LGRB Metallicity Evolution with Redshift

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

Gamma-ray bursts and supernovae4 citations

TL;DR

This study analyzes long-duration gamma-ray burst (LGRB) host galaxies up to redshift z = 2.5 and finds no significant evolution in their metallicity distribution, with ~25% of hosts maintaining high metallicity (12+log(O/H) > 8.4) across all redshifts. The result contradicts expectations from the evolving mass-metallicity relation in typical galaxies, suggesting LGRB hosts are systematically more metal-rich than predicted by standard scaling relations, resolving prior discrepancies in metallicity cutoff estimates.

ABSTRACT

Recent additions to the population of Long-duration Gamma Ray Burst (LGRB) host galaxies with measured metallicities and host masses allow us to investigate how the distributions of both these properties change with redshift. We form a sample out to z of 2.5 which we show does not have strong redshift dependent populations biases in mass and metallicity measurements. Using this sample, we find a surprising lack of evolution in the LGRB metallicity distribution across different redshifts and in particular the fraction of LGRB hosts with relatively high-metallicity, that is those with 12+log(O/H) > 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. By converting the measured LGRB host masses and redshifts to expected metallicities using redshift appropriate mass-metallicity relations, we further find that the increase in LGRB host galaxy mass distribution with redshift seen in the Perley et al. (2016) SHOALS sample is consistent with that needed to preserve a non-evolving LGRB metallicity distribution. However, the estimated LGRB host metallicity distribution is at least a quarter dex higher at all redshifts than the measured metallicity distribution. This corresponds to about a factor of two in raw metallicity and resolves much of the difference between the LGRB host metallicity cutoffs determined by Graham & Fruchter (2017) and Perley et al. (2016). As LGRB hosts do not follow the general mass metallicity relations, there is no substitute for actually measuring their metallicities.

Motivation & Objective

• To investigate whether the metallicity distribution of long-duration gamma-ray burst (LGRB) host galaxies evolves with redshift up to z = 2.5.
• To assess whether selection biases in mass and metallicity measurements affect the reliability of the LGRB host sample.
• To compare observed LGRB host metallicities with predictions from the standard galaxy mass-metallicity relation at different redshifts.
• To resolve discrepancies in reported LGRB host metallicity cutoffs between previous studies (Graham & Fruchter 2017; Perley et al. 2016).

Proposed method

• Constructed a sample of LGRB host galaxies with measured metallicities and host masses up to redshift z = 2.5, ensuring minimal redshift-dependent biases in mass and metallicity measurements.
• Used redshift-appropriate mass-metallicity relations to predict expected metallicities for LGRB hosts based on their measured masses and redshifts.
• Compared the observed LGRB host metallicity distribution with both the predicted metallicities from the mass-metallicity relation and the observed distributions in typical galaxies.
• Quantified the offset between observed LGRB host metallicities and predicted values, finding a consistent ~0.25 dex higher metallicity at all redshifts.
• Evaluated the consistency of the observed mass evolution in the Perley et al. (2016) SHOALS sample with the non-evolving metallicity distribution.

Experimental results

Research questions

• RQ1 Does the metallicity distribution of LGRB host galaxies evolve with redshift up to z = 2.5?
• RQ2 How do the observed metallicities of LGRB hosts compare to those predicted by the standard galaxy mass-metallicity relation at different redshifts?
• RQ3 Why do previous studies report conflicting metallicity cutoffs for LGRB hosts?
• RQ4 Is the observed increase in LGRB host mass with redshift sufficient to explain the lack of metallicity evolution?
• RQ5 To what extent do LGRB hosts deviate from the general mass-metallicity relation in typical galaxies?

Key findings

• The fraction of LGRB hosts with high metallicity (12+log(O/H) > 8.4) remains essentially constant at ~25% from z = 0 to z = 2.5, indicating no significant metallicity evolution.
• The observed LGRB host metallicity distribution is systematically higher by at least 0.25 dex (a factor of ~2 in raw metallicity) than the values predicted by the standard mass-metallicity relation at each redshift.
• The observed increase in LGRB host mass with redshift, as reported in the Perley et al. (2016) SHOALS sample, is sufficient to account for the non-evolving metallicity distribution when combined with the redshift-dependent mass-metallicity relation.
• The observed offset between actual and predicted metallicities resolves much of the discrepancy between earlier estimates of LGRB host metallicity cutoffs by Graham & Fruchter (2017) and Perley et al. (2016).
• LGRB hosts do not follow the general mass-metallicity relation observed in typical galaxies, indicating that direct metallicity measurements are essential for accurate characterization.
• The lack of metallicity evolution in LGRB hosts cannot be explained by selection effects in mass or metallicity measurements, as the sample shows no strong redshift-dependent biases.