Near-infrared Spectroscopy of GRB Host Galaxies at z >~ 1.5: Insights
  into Host Galaxy Dynamics and Interpretations of Afterglow Absorption Spectra
  This paper presents near-infrared echellette spectra of faint galaxies in the
fields around GRB 050820A at redshift z=2.613 and GRB 060418 at z=1.490. The
spectroscopic data show that both GRBs originate in a dynamic environment of
interacting galaxies separated by < 15 h^{-1} kpc in projected distance and
|dv| <~ 60 km/s in line-of-sight velocity. The optical afterglows revealed in
early-epoch Hubble Space Telescope images are at least 2.5 h^{-1} kpc (or 0.4")
away from the high surface brightness regions of the interacting members,
indicating that the GRB events occurred either in the outskirts of a compact
star-forming galaxy or in a low surface brightness satellite. Comparisons of
the systemic redshifts of the host galaxies and the velocity distribution of
absorbing clouds revealed in early-time afterglow spectra further show that the
majority of the absorbing clouds are redshifted from these compact star-forming
galaxies. These include the gas producing fine-structure absorption lines at
physical distances d ~ a few x 100 pc from the GRB afterglow. The lack of
blueshifted absorbing clouds and the spatial offset of the GRB event from the
star-forming regions make it difficult to attribute the observed large velocity
spread (~ 200-400 km/s) of absorbing gas in the GRB host to galactic-scale
outflows. We consider a scenario in which the GRB event occurred in a dwarf
satellite of the interacting group and interpret the broad absorption
signatures in the afterglow spectra as a collective effect of the turbulent
halo gas and the host star-forming ISM. We briefly discuss the implications for
the absorption properties observed in the afterglow spectra.
