A new population of high-redshift short-duration gamma-ray bursts

The redshift distribution of the short-duration gamma-ray bursts (GRBs) is a crucial, but currently fragmentary, clue to the nature of their progenitors. Her ewe present optical observations of nine short GRBs obtained with Gemini, Magellan, and the Hubble Space Telescope. We detect the afterglows and host galaxies of two short bursts, and host galaxies for two additional bursts with known optical afterglow positions, and five with X-ray positions (less than or similar to 6 '' radius). In eight of the nine cases we find that the most probable host galaxies are faint, R approximate to 23-26.5 mag, and are therefore starkly different from the first few short GRB hosts with R approximate to 17-22 mag and z less than or similar to 0.5. Indeed, we measure spectroscopic redshifts of z approximate to 0.4-1.1 for the four brightest hosts. A comparison to large field galaxy samples, as well as the hosts of long GRBs and previous short GRBs, indicates that the fainter hosts likely reside at z greater than or similar to 1. Our most conservative limit is that at least half of the five hosts without a known redshift reside at z > 0.7 (97% confidence level), suggesting that about 1/3 to 2/3 of all short GRBs originate at higher redshifts than previously determined. This has two important implications: (1) we constrain the acceptable age distributions to a wide lognormal (sigma greater than or similar to 1) with tau(*)similar to 4-8 Gyr, or to a power law, P(tau) proportional to tau(n), with -1 less than or similar to n less than or similar to 0; and ( 2) the inferred isotropic energies, E-gamma,E-iso similar to 10(50)-10(52) ergs, are significantly larger than similar to 10(48)-10(49) ergs for the low-redshift, short GRBs, indicating a large spread in energy release or jet opening angles. Finally, we reiterate the importance of short GRBs as potential gravitational-wave sources and find a conservative detection rate with the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) of similar to 2-6 yr(-1).