The GRB Host Galaxies and Redshifts Based in part on the observations obtained at the W.M. Keck Observatory, operated by the California Association for Research in Astronomy, a scientific partnership among Caltech, the Univ. of California and NASA; and with the NASA/ESA Hubble Space Telescope, operated by the AURA, Inc., under a contract with NASA.

Abstract.

Observations of GRB host galaxies and their environments in general can provide valuable clues about the nature of progenitors. Bursts are associated with faint, $\langle R\rangle \sim 25$ mag, galaxies at cosmological redshifts, $\langle z\rangle \sim 1$. The host galaxies span a range of luminosities and morphologies, and appear to be broadly typical for the normal, evolving, actively star-forming galaxy populations at comparable redshifts and magnitudes, but may have somewhat elevated SFR per unit luminosity. There are also spectroscopic hints of massive star formation, from the ratios of [Ne III] and [O II] lines. The observed, unobscured star formation rates are typically a few $M_\odot /$yr, but a considerable fraction of the total star formation in the hosts may be obscured by dust. A census of detected optical afterglows provides a powerful new handle on the obscured fraction of star formation in the universe; the current results suggest that at most a half of the massive star formation was hidden by dust.