Near-infrared Spectroscopy of GRB Host Galaxies at z >~ 1.5: Insights into Host Galaxy Dynamics and Interpretations of Afterglow Absorption Spectra

TL;DR

This study uses near-infrared spectroscopy to analyze the environments of two high-redshift GRB host galaxies, revealing complex dynamics, galaxy interactions, and insights into the origin of absorption features in afterglow spectra.

Contribution

It provides new spectroscopic evidence of galaxy interactions and the spatial offset of GRBs from star-forming regions at z > 1.5, enhancing understanding of GRB host environments.

Findings

GRBs are in interacting galaxy systems separated by <15 h^{-1} kpc.

GRB locations are offset from high surface brightness regions.

Absorbing clouds are mostly redshifted, not outflowing.

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 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.