A Population of Massive, Luminous Galaxies Hosting Heavily Dust-Obscured
  Gamma-Ray Bursts: Implications for the Use of GRBs as Tracers of Cosmic Star
  Formation
  We present observations and analysis of the host galaxies of 23 heavily
dust-obscured gamma-ray bursts (GRBs) observed by the Swift satellite during
the years 2005-2009, representing all GRBs with an unambiguous host-frame
extinction of A_V>1 mag from this period. Deep observations with Keck, Gemini,
VLT, HST, and Spitzer successfully detect the host galaxies and establish
spectroscopic or photometric redshifts for all 23 events, enabling us to
provide measurements of the intrinsic host star-formation rates, stellar
masses, and mean extinctions. Compared to the hosts of unobscured GRBs at
similar redshifts, we find that the hosts of dust-obscured GRBs are (on
average) more massive by about an order of magnitude and also more rapidly
star-forming and dust-obscured. While this demonstrates that GRBs populate all
types of star-forming galaxies including the most massive, luminous systems at
z~2, at redshifts below 1.5 the overall GRB population continues to show a
highly significant aversion away from massive galaxies and a preference for
low-mass systems relative to what would be expected given a purely SFR-selected
galaxy sample. This supports the notion that the GRB rate is strongly dependent
on metallicity, and may suggest that the most massive galaxies in the Universe
underwent a transition in their chemical properties ~9 Gyr ago. We also
conclude that, based on the absence of unobscured GRBs in massive galaxies and
the absence of obscured GRBs in low-mass galaxies, the dust distributions of
the lowest-mass and the highest-mass galaxies are relatively homogeneous, while
intermediate-mass galaxies (~10^9 M_sun) have diverse internal properties.
