DEMOGRAPHICS OF THE GALAXIES HOSTING SHORT-DURATION GAMMA-RAY BURSTS

We present observations of the afterglows and host galaxies of three short-duration gamma-ray bursts (GRBs): 100625A, 101219A, and 110112A. We find that GRB 100625A occurred in a z = 0.452 early-type galaxy with a stellar mass of approximate to 4.6 x 10(9) M-circle dot and a stellar population age of approximate to 0.7 Gyr, and GRB 101219A originated in a star-forming galaxy at z = 0.718 with a stellar mass of approximate to 1.4 x 10(9) M-circle dot, a star formation rate of approximate to 16 M-circle dot yr(-1), and a stellar population age of approximate to 50 Myr. We also report the discovery of the optical afterglow of GRB 110112A, which lacks a coincident host galaxy to i greater than or similar to 26 mag, and we cannot conclusively identify any field galaxy as a possible host. From afterglow modeling, the bursts have inferred circumburst densities of approximate to 10(-4)-1 cm(-3) and isotropic-equivalent gamma-ray and kinetic energies of approximate to 10(50)-10(51) erg. These three events highlight the diversity of galactic environments that host short GRBs. To quantify this diversity, we use the sample of 36 Swift short GRBs with robust associations to an environment (similar to 1/2 of 68 short bursts detected by Swift to 2012 May) and classify bursts originating from four types of environments: late-type (approximate to 50%), early-type (approximate to 15%), inconclusive (approximate to 20%), and "host-less" (lacking a coincident host galaxy to limits of greater than or similar to 26 mag; approximate to 15%). To find likely ranges for the true late- and early-type fractions, we assign each of the host-less bursts to either the late- or early-type category using probabilistic arguments and consider the scenario that all hosts in the inconclusive category are early-type galaxies to set an upper bound on the early-type fraction. We calculate most likely ranges for the late- and early-type fractions of approximate to 60%-80% and approximate to 20%-40%, respectively. We find no clear trend between gamma- ray duration and host type. We also find no change to the fractions when excluding events recently claimed as possible contaminants from the long GRB/collapsar population. Our reported demographics are consistent with a short GRB rate driven by both stellar mass and star formation.