Unveiling the Secrets of Metallicity and Massive Star Formation Using
  DLAs along Gamma-ray Bursts
  We present the largest, publicly available, sample of Damped Lyman-$\alpha$
systems (DLAs) along Gamma-ray Bursts (GRB) line of sights in order to
investigate the environmental properties of long GRBs in the $z=1.8-6$ redshift
range. Compared with the most recent quasar DLAs sample (QSO-DLA), our analysis
shows that GRB-DLAs probe a more metal enriched environment at $z\gtrsim3$, up
to $[X/H]\sim-0.5$. In the $z=2-3$ redshift range, despite the large number of
lower limits, there are hints that the two populations may be more similar
(only at 90\% significance level). Also at \hiz, the GRB-DLA average
metallicity seems to decline at a shallower rate than the QSO-DLAs: GRB-DLA
hosts may be polluted with metals at least as far as $\sim 2$kpc from the GRB
explosion site, probably due to previous star-formation episodes and/or
supernovae explosions. This shallow metallicity trend, extended now up to
$z\sim5$, confirms previous results that GRB hosts are star-forming and have,
on average, higher metallicity than the general QSO-DLA population. Finally,
our metallicity measurements are broadly consistent with the hypothesis of two
channels of GRB progenitors, one of which is mildly affected by a metallicity
bias. The metallicity evolution of modeled GRB hosts agrees reasonably well
with our data up to intermediate redshift, while more data are needed to
constrain the models at $z\gtrsim 4$.
