Exploring dust extinction at the edge of reionization
  The brightness of gamma-ray burst (GRB) afterglows and their occurrence in
young, blue galaxies make them excellent probes to study star forming regions
in the distant Universe. We here elucidate dust extinction properties in the
early Universe through the analysis of the afterglows of all known z > 6 GRBs:
GRB 090423, 080913 and 050904, at z = 8.2, 6.69, and 6.295, respectively. We
gather all available optical and near-infrared photometry, spectroscopy and
X-ray data to construct spectral energy distributions (SEDs) at multiple
epochs. We then fit the SEDs at all epochs with a dust-attenuated power-law or
broken power-law. We find no evidence for dust extinction in GRB 050904 and GRB
090423, with possible evidence for a low level of extinction in GRB 080913. We
compare the high redshift GRBs to a sample of lower redshift GRB extinctions
and find a lack of even moderately extinguished events (AV ~ 0.3) above z > 4.
In spite of the biased selection and small number statistics, this result hints
at a decrease in dust content in star-forming environments at high redshifts.
