A search for hostless short GRBs with large-aperture telescopes

The environment of short duration gamma-ray bursts (sGRBs) yields important information as to their progenitors and their formation channels. In particular, the location of sGRBs around their host galaxies — the so-called offset distribution — depends on the age and natal kick velocity of the progenitor system. A significant fraction (20-30%) of sGRBs lack a coincident host galaxy to deep limits (r>26 AB mag) and are found significantly offset from any other potential host galaxy. These hostless sGRBs are a direct prediction of the compact binary merger model, and their rate improves our understanding of the formation and evolution of compact binaries, complementing the constraints derived from GW astronomy. The identification of hostless sGRBs requires an accurate GRB localization as well as sensitive optical and near-infrared imaging to detect any faint underlying host galaxy. For this reason, we started a systematic search for these events targeting all well-localized sGRBs without a known host galaxy association. Here, we present the results of our observing campaign, including deep Gemini, Keck and HST imaging of 30 new short GRBs. Our study effectively doubles the sample of well studied sGRB galaxies and represents the largest catalog to date of sGRB offsets.