A Surprising Lack of LGRB Metallicity Evolution with Redshift
  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.
