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. Here we 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 (≲6'' radius). In eight of the nine cases we find that the most probable host galaxies are faint, R ≈ 23-26.5 mag, and are therefore starkly different from the first few short GRB hosts with R ≈ 17-22 mag and z ≲ 0.5. Indeed, we measure spectroscopic redshifts of z ≈ 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 ≳ 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 to 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 (σ ≳ 1) with τ * ~ 4-8 Gyr, or to a power law, P (τ) ∝ τ n , with -1 ≲ n ≲ 0; and (2) the inferred isotropic energies, E γ,iso ~ 10 50 -10 52 ergs, are significantly larger than ~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 ~2-6 yr -1 .