Interpreting Short Gamma Ray Burst Progenitor Kicks and Time Delays
  Using the Host Galaxy-Dark Matter Halo Connection
  Nearly 20% of short gamma-ray bursts (sGRBs) have no observed host galaxies.
Combining this finding with constraints on galaxies' dark matter halo potential
wells gives strong limits on the natal kick velocity distribution for sGRB
progenitors. For the best-fitting velocity distribution, one in five sGRB
progenitors receives a natal kick above 150 km/s, consistent with merging
neutron star models but not with merging white dwarf binary models. This
progenitor model constraint is robust to a wide variety of systematic
uncertainties, including the sGRB progenitor time-delay model, the Swift
redshift sensitivity, and the shape of the natal kick velocity distribution. We
also use constraints on the galaxy-halo connection to determine the host halo
and host galaxy demographics for sGRBs, which match extremely well with
available data. Most sGRBs are expected to occur in halos near 10^12 Msun and
in galaxies near 5x10^10 Msun (L_*); unobserved faint and high-redshift host
galaxies contribute a small minority of the observed hostless sGRB fraction. We
find that sGRB redshift distributions and host galaxy stellar masses weakly
constrain the progenitor time-delay model; the active vs. passive fraction of
sGRB host galaxies may offer a stronger constraint. Finally, we discuss how
searches for gravitational wave optical counterparts in the local Universe can
reduce followup times using these findings.
