The luminosity and stellar mass functions of GRB host galaxies: Insight
  into the metallicity bias
  Long-Duration Gamma-Ray Bursts (GRBs) are powerful probes of the Universe
star formation history, but correlation between the two depends on the highly
debated presence/strength of a metallicity bias. To investigate this
correlation, we use a phenomenological model that successfully describes star
formation rates, luminosities and stellar masses of star forming galaxies,
applying it to GRB production. We predict luminosities, stellar masses, and
metallicities of host galaxies depending on the metallicity bias. Our
best-fitting model includes a moderate metallicity bias, broadly consistent
with the large majority of long-duration GRBs in metal-poor environments
originating from collapsars (probability ~83%), but with a secondary
contribution (~17%) from metal-independent production channels, such as binary
evolution. Because of the mass-metallicity relation of galaxies, the maximum
likelihood model predicts that the metal-independent channel becomes dominant
at z<2, where hosts have higher metallicities and collapsars are suppressed.
This possibly explains why some studies find no clear evidence of a metal-bias
based on low-z samples. However, while metallicity predictions match
observations well at high redshift, there is tension with low redshift
observations, since a significant fraction of GRB hosts are predicted to have
(near-)solar metallicity. This is in contrast to observations, unless obscured,
metal-rich hosts are preferentially missed in current datasets, and suggests
that lower efficiencies of the metal-independent GRB channel might be preferred
following a comprehensive fit from complete samples. Overall, we are able to
establish the presence of a metallicity bias for GRB production, but continued
characterization of GRB host galaxies is needed to quantify its strength.
