Abstract
We present observations and analysis of the host galaxies of 23 heavily
dust-obscured gamma-ray bursts (GRBs) observed by the Swift satellite between
the years 2005-2009, a sample representing all GRBs during this period with an
unambiguous host-frame extinction of at least A_V>1 mag. Deep observations with
Keck, Gemini, VLT, HST, and Spitzer successfully detect the host galaxies and
establish redshifts for all 23 events, enabling us to provide measurements of
the host stellar masses, star-formation rates (SFRs), 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 are also significantly more rapidly star-forming and more
dust-obscured. However, while the inclusion of this population of dust-obscured
hosts shows that GRBs populate all types of star-forming galaxies including the
most massive, luminous systems at z~2, at redshifts below z<1.5 the overall GRB
population continues to show a highly significant aversion away from massive
galaxies and a corresponding preference for low-mass systems relative to 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 an significant transition
in their chemical properties approximately 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 both the
lowest-mass and the highest-mass galaxies are relatively homogeneous, while
intermediate-mass galaxies (~10^9 M_sun) have much more diverse internal dust
properties.
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