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ABSTRACT  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 gamma‐ray bursts (GRBs) originate in a dynamic environment of interacting galaxies separated by &lt;15  h  −1  kpc in projected distance and |Δ v | ≲ 60   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 arcsec) 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 ∼ afew × 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  ) 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 interstellar medium. We briefly discuss the implications for the absorption properties observed in the afterglow spectra.

monthly notices of the royal astronomical society

Near‐infrared spectroscopy of gamma‐ray burst host galaxies at   : insights into host galaxy dynamics and interpretations of afterglow absorption spectra