The galaxy hosts and large-scale environments of short-hard (gamma)-ray bursts

The nature of the progenitors of short duration, hard spectrum, gamma-ray bursts (GRBs) has remained a mystery. Even with the recent localizations of four short-hard GRBs, no transient emission has been found at long wavelengths that directly constrains the progenitor nature. Instead, as was the case in studying the different morphological subclasses of supernovae and the progenitors of long-duration GRBs, we suggest that the progenitors of short bursts can be meaningfully constrained by the environment in which the bursts occur. Here we present the discovery spectra of the galaxies that hosted three short-hard GRBs and the spectrum of a fourth host. The results indicate that these environments, both at the galaxy scale and galaxy-cluster scale, differ substantially from those of long-soft GRBs. The spatial offset of three bursts from old and massive galaxy hosts strongly favors an origin from the merger of compact stellar remnants, such as double neutron stars or a neutron-star black hole binary. The star-forming host of another GRB provides confirmation that, like supernovae of Type Ia, the progenitors of short-hard bursts are created in all galaxy types. This indicates a class of progenitors with a wide distribution of delay times between formation and explosion