Gas inflow and outflow in an interacting high-redshift galaxy: The
  remarkable host environment of GRB 080810 at $z=3.35$
  We reveal multiple components of an interacting galaxy system at
$z\approx3.35$ through a detailed analysis of the exquisite high-resolution
Keck/HIRES spectrum of the afterglow of a gamma-ray burst (GRB). Through
Voigt-profile fitting of absorption lines from the Lyman-series, we constrain
the neutral hydrogen column density to $N_{\mathrm{HI}} \leq 10^{18.35}$
cm$^{-2}$ for the densest of four distinct systems at the host redshift of
GRB~080810, among the lowest $N_{\mathrm{HI}}$ ever observed in a GRB host,
despite the line of sight passing within a projected 5 kpc of the galaxy
centres. By detailed analysis of the corresponding metal absorption lines, we
derive chemical, ionic and kinematic properties of the individual absorbing
systems, and thus build a picture of the host as a whole. Striking differences
between the systems imply that the line of sight passes through several phases
of gas: the star-forming regions of the GRB host; enriched material in the form
of a galactic outflow; the hot and ionised halo of a second, interacting galaxy
falling towards the host at a line-of-sight velocity of 700 km s$^{-1}$; and a
cool, metal-poor cloud which may represent one of the best candidates yet for
the inflow of metal-poor gas from the intergalactic medium.
