On the mass-metallicity relation, velocity dispersion and gravitational well depth of GRB host galaxies

TL;DR

This study compares the metallicity and velocity dispersion of GRB host galaxies with QSO-DLAs, finding they are consistent in properties and evolution, and explores how impact parameters influence observed metallicities and velocity fields.

Contribution

It demonstrates that GRB host galaxies and QSO-DLAs are likely from the same population and analyzes the effects of impact parameters on observed properties.

Findings

GRB host galaxies and QSO-DLAs have similar metallicity-velocity relations.

The redshift evolution of the mass-metallicity relation favors a late onset around z≈2.6.

Evidence suggests impact parameter effects influence observed metallicity and velocity measurements.

Abstract

We analyze a sample of 16 absorption systems intrinsic to long duration GRB host galaxies at $z \gtrsim 2$ for which the metallicities are known. We compare the relation between the metallicity and cold gas velocity width for this sample to that of the QSO-DLAs, and find complete agreement. We then compare the redshift evolution of the mass-metallicity relation of our sample to that of QSO-DLAs and find that also GRB hosts favour a late onset of this evolution, around a redshift of $\approx 2.6$. We compute predicted stellar masses for the GRB host galaxies using the prescription determined from QSO-DLA samples and compare the measured stellar masses for the four hosts where stellar masses have been determined from SED fits. We find excellent agreement and conclude that, on basis of all available data and tests, long duration GRB-DLA hosts and intervening QSO-DLAs are consistent with being drawn from the same underlying population. GRB host galaxies and QSO-DLAs are found to have different impact parameter distributions and we briefly discuss how this may affect statistical samples. The impact parameter distribution has two effects. First any metallicity gradient will shift the measured metallicity away from the metallicity in the centre of the galaxy, second the path of the sightline through different parts of the potential well of the dark matter halo will cause different velocity fields to be sampled. We report evidence suggesting that this second effect may have been detected.