A New Population of High Redshift Short-Duration Gamma-Ray Bursts

The redshift distribution of the short-duration GRBs is a crucial, but currently fragmentary, clue to the nature of their progenitors. Here we present optical observations of seven short GRBs obtained with Gemini, Magellan, and HST. We detect the afterglows and host galaxies of two short bursts, and host galaxies for two additional bursts with known optical afterglow positions, and three with X-ray positions (<4.5" radius). In all seven cases we find that the most probable host galaxies are faint, R=23-26.5 mag, and therefore starkly different from the first few short GRB hosts, with R=17-22 mag and z<0.5. A comparison to large field galaxy samples, as well as the hosts of long GRBs and the previous short GRBs, indicates that these new hosts likely reside at z~1, and in fact we measure a spectroscopic redshift of z=1.1304 for the putative host of GRB060801. Our most conservative limit is that at least half of this new sample resides at z>0.7 (97% confidence level), suggesting that 1/4-2/3 of all short GRBs originate at higher redshifts than previously determined. This has two important implications: (i) We constrain the acceptable age distributions to a wide lognormal (sigma~1) with tau~4-8 Gyr, or to a power law, P(tau)~tau^n, with -1<n<0; and (ii) the inferred isotropic energies, E_{gamma,iso}~10^{50}-10^{52} erg, are significantly larger than 10^{48}-10^{49} erg for the low redshift short GRBs, indicating a large spread in energy release or jet opening angles. Finally, we re-iterate the importance of short GRBs as potential gravitational wave sources and find a conservative Advanced LIGO rate of ~2-6 per year.