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

We analyse a sample of 16 absorption systems intrinsic to long-duration gamma-ray burst (GRB) host galaxies at z ≳ 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 (quasi-stellar object-damped Lyman α), 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 ≈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 spectral energy distribution (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, and secondly, 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.