The GRB Host Galaxies and Redshifts

Observations of GRB host galaxies and their environments in general can provide valuable clues about the nature of progenitors. Bursts are associated with faint, � R �∼ 25 mag, galaxies at cosmological redshifts, � z �∼ 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/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.