The Locations of Short Gamma-ray Bursts as Evidence for Compact Object Binary Progenitors

We present a detailed investigation of Hubble Space Telescope (HST) rest-frame UV/optical observations of 22 short gamma-ray burst (GRB) host galaxies and sub-galactic environments. Utilizing the high angular resolution and depth of HST, we characterize the host galaxy morphologies, measure precise projected physical and host-normalized offsets between the bursts and host centers, and calculate the locations of the bursts with respect to their host light distributions (rest-frame UV and optical). We calculate a median short GRB projected physical offset of 4.5 kpc, about 3.5 times larger than that for long GRBs, and find that ~25% of short GRBs have offsets of >10 kpc. When compared to their host sizes, the median offset is 1.5 half-light radii (r_e), about 1.5 times larger than the values for long GRBs, core-collapse supernovae, and Type Ia supernovae. In addition, ~20% of short GRBs having offsets of >5 r_e, and only ~25% are located within 1 r_e. We further find that short GRBs severely under-represent their hosts' rest-frame optical and UV light, with ~30-45% of the bursts located in regions of their host galaxies that have no detectable stellar light, and ~55% in the regions with no UV light. Therefore, short GRBs do not occur in regions of star formation or even stellar mass. This demonstrates that the progenitor systems of short GRBs must migrate from their birth sites to their eventual explosion sites, a signature of kicks in compact object binary systems. Utilizing the full sample of offsets, we estimate natal kick velocities of ~20-140 km s^-1. These independent lines of evidence provide the strongest support to date that short GRBs result from the merger of compact object binaries (NS-NS/NS-BH).