The Swift Gamma-Ray Burst Host Galaxy Legacy Survey - I. Sample
  Selection and Redshift Distribution
  We introduce the Swift Gamma-Ray Burst Host Galaxy Legacy Survey ("SHOALS"),
a multi-observatory high-redshift galaxy survey targeting the largest unbiased
sample of long-duration gamma-ray burst hosts yet assembled (119 in total). We
describe the motivations of the survey and the development of our selection
criteria, including an assessment of the impact of various observability
metrics on the success rate of afterglow-based redshift measurement. We briefly
outline our host-galaxy observational program, consisting of deep Spitzer/IRAC
imaging of every field supplemented by similarly-deep, multi-color optical/NIR
photometry, plus spectroscopy of events without pre-existing redshifts. Our
optimized selection cuts combined with host-galaxy follow-up have so far
enabled redshift measurements for 110 targets (92%) and placed upper limits on
all but one of the remainder. About 20% of GRBs in the sample are heavily
dust-obscured, and at most 2% originate from z>5.5. Using this sample we
estimate the redshift-dependent GRB rate density, showing it to peak at z~2.5
and fall by about an order of magnitude towards low (z=0) redshift, while
declining more gradually towards high (z~7) redshift. This behavior is
consistent with a progenitor whose formation efficiency varies modestly over
cosmic history. Our survey will permit the most detailed examination to date of
the connection between the GRB host population and general star-forming
galaxies, directly measure evolution in the host population over cosmic time
and discern its causes, and provide new constraints on the fraction of cosmic
star-formation occurring in undetectable galaxies at all redshifts.
