The Host Galaxies of Swift Dark Gamma-Ray Bursts: Observational
  Constraints on Highly Obscured and Very High-Redshift GRBs
  In this work we present the first results of our imaging campaign at Keck
Observatory to identify the host galaxies of "dark" gamma-ray bursts (GRBs),
events with no detected optical afterglow or with detected optical flux
significantly fainter than expected from the observed X-ray afterglow. We find
that out of a uniform sample of 29 Swift bursts rapidly observed by the Palomar
60-inch telescope through March 2008 (14 of which we classify as dark), all
events have either a detected optical afterglow, a probable optical host-galaxy
detection, or both. Our results constrain the fraction of Swift GRBs coming
from very high redshift (z > 7), such as the recent GRB 090423, to between
0.2-7 percent at 80% confidence. In contrast, a significant fraction of the
sample requires large extinction columns (host-frame A_V > 1 mag, with several
events showing A_V > 2-6 mag), identifying dust extinction as the dominant
cause of the dark GRB phenomenon. We infer that a significant fraction of GRBs
(and, by association, of high-mass star formation) occurs in highly obscured
regions. However, the host galaxies of dark GRBs seem to have normal optical
colors, suggesting that the source of obscuring dust is local to the vicinity
of the GRB progenitor or highly unevenly distributed within the host galaxy.
