Short GRB Host Galaxies. II. A Legacy Sample of Redshifts, Stellar Population Properties, and Implications for Their Neutron Star Merger Origins

We present the stellar population properties of 69 short gamma-ray burst (GRB) host galaxies, representing the largest uniformly modeled sample to date. Using the Prospector stellar population inference code, we jointly fit photometry and/or spectroscopy of each host galaxy. We find a population median redshift of z = 0.64(-0.32)(+0.83) (68% confidence), including nine photometric redshifts at z greater than or similar to 1. We further find a median mass-weighted age of t(m) = 0.8(-0.53)(+2.71) Gyr, stellar mass of log(M-*/M-circle dot)= 9.69(-0.65)(+0.75), star formation rate of SFR =1.44(-1.35)(+9.37) M(circle dot)yr(-1), stellar metallicity of log(Z(*)/Z(circle dot)) = - 0.38(-0.42)(+0.44) and dust attenuation of A(V) = 0.43(-0.36)(+0.85) mag (68% confidence). Overall, the majority of short GRB hosts are star-forming (approximate to 84%), with small fractions that are either transitioning or quiescent (approximate to 10%); however, we observe a much larger fraction (approximate to 40%) of quiescent and transitioning hosts at z less than or similar to 0.25, commensurate with galaxy evolution. We find that short GRB hosts populate the star-forming main sequence of normal field galaxies, but do not include as many high-mass galaxies as the general galaxy population, implying that their binary neutron star (BNS) merger progenitors are dependent on a combination of host star formation and stellar mass. The distribution of ages and redshifts implies a broad delay-time distribution, with a fast-merging channel at z > 1 and a decreased neutron star binary formation efficiency from high to low redshifts. If short GRB hosts are representative of BNS merger hosts within the horizon of current gravitational wave detectors, these results can inform future searches for electromagnetic counterparts. All of the data and modeling products are available on the Broadband Repository for Investigating Gamma-ray burst Host Traits website.