Are LGRBs biased tracers of star formation? Clues from the host galaxies
  of the Swift/BAT6 complete sample of bright LGRBs. II: star formation rates
  and metallicities at z < 1
  Long gamma-ray bursts (LGRBs) are associated with the deaths of massive stars
and could thus be a potentially powerful tool to trace cosmic star formation.
However, especially at low redshifts (z < 1.5) LGRBs seem to prefer particular
types of environment. Our aim is to study the host galaxies of a complete
sample of bright LGRBs to investigate the impact of the environment on GRB
formation. We study host galaxy spectra of the Swift/BAT6 complete sample of 14
z < 1 bright LGRBs. We use the detected nebular emission lines to measure the
dust extinction, star formation rate (SFR) and nebular metallicity (Z) of the
hosts and supplement the data set with previously measured stellar masses
M$_{\star}$. The distributions of the obtained properties and their
interrelations (e.g. mass-metallicity and SFR-M$_{\star}$ relations) are
compared to samples of field star-forming galaxies.We find that LGRB hosts at z
< 1 have on average lower SFRs than if they were direct star-formation tracers.
By directly comparing metallicity distributions of LGRB hosts and star-forming
galaxies, we find a good match between the two populations up to Z $\sim
8.4-8.5$, after which the paucity of metal-rich LGRB hosts becomes apparent.
The LGRB host galaxies of our complete sample are not inconsistent with the
mass-metallicity relation at similar mean redshift and stellar masses. The
cutoff against high metallicities (and high masses) can explain the low SFR
values of LGRB hosts. We find a hint of increased incidence of starburst
galaxies in the Swift/BAT6 z < 1 sample with respect to that of a field
star-forming population. Given that the SFRs are low on average, the latter is
ascribed to low stellar masses. Nevertheless the limits on the completeness and
metallicity availability of current surveys, coupled with the limited number of
LGRB host galaxies, prevent us from reaching more quantitative conclusions.
