During the past decade extraordinary progress has been made in determining the origin of long-duration gamma-ray bursts. It has been conclusively shown that these objects derive from the deaths of massive stars. Nonetheless, the origin of their observational cousins, short-duration gamma-ray bursts (SGRBs) remains a mystery. While SGRBs are widely thought to result from the inspiral of compact binaries, this is a conjecture. A number of hosts of SGRBs have been identified, and have been used by some to argue that SGRBs derive primarily from an ancient population (~ 5 Gyr); however, it is not known whether this conclusion more accurately reflects selection biases or astrophysics. Here we propose to employ a variant of a technique that we pioneered and used to great effect in elucidating the origins of long-duration bursts. We will examine the degree to which SGRB locations trace the red or blue light of their hosts, and thus old or young stellar populations. This approach will allow us to study the demographics of the SGRB population in a manner largely free of the distance dependent selection effects which have so far bedeviled this field, and should give direct insight into the age of the SGRB progenitor population.

We propose to observe SGRB hosts where the location of the GRB is known to ~0".1. In practice, these are GRBs with optical transients (OTs). A number of these objects have already been observed with HST, primarily in F814W, and in cases where some of these images were taken short after the burst, the HST image will give a precise position. In other cases, early Gemini and/or VLT images will be available from their archives during Cycle 16. We expect these images to provide locations with a one sigma error of ~0".05. In cases where a redshift is known, we have chosen filters that will straddle the 4000 A break. In other cases (primarily cases where the host magnitude is greater than 25), we have assumed that the redshift is comparable to or greater than one, and have chosen filters which should assure that we see the 4000 A break. Our observations will reach a signal-to-noise ratio of greater than 1 per WFPC2 pixel at a surface brightness between 25 and 25.5 mag AB per square arcsecond. This is true in all filters, on the WFPC2, and in the equivalent drizzled area in NIC3 images. We used a SNR of 1 as the definition of the edge of the host in the HST images analyzed in our Nature paper on burst locations. Most of these images reached this limit in either the STIS (open) or ACS F606W in native pixels around a surface brightness of 25 AB mag per square arcsecond. Thus we would reach a similar depth though with slightly reduced resolution. We plan to use the NIC3 rather than the more finely sampled NIC2 in order to assure that we can perform the astrometry required to accurately place a GRB on its host. One object in our sample, GRB 061201, is offset from the center of Abell Cluster S0995 at z = 0.0835 by about 1 Mpc, assuming the object is at the distance of the cluster; however, no host has been identified from ground. HST observations were taken by Fox et al. as a ToO, but they have yet to release a circular. Depending on the results of these observations out choice of filter (and potentially camera) for this object could change. The HST observations could also potentially provide a highly accurate position, though a position to better than a single WFPC2 could be obtained from the archival Gemini image. If no host is found in the existing HST images, a deep NIR search with F160W would be valuable to determine if the location of the object might be due to a chance superposition. In this case, a distant red host might exist. If no host is found in the NIR, it would suggest this object was formed from either a star or globular cluster in the "intracluster light". 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