Dust is an important tracer of chemical enrichment in primeval galaxies and it has also important implications for their evolution .
So far , at z > 6 , close to the re-ionization epoch , the presence of dust has been only firmly established in quasar host galaxies , which are rare objects associated with enormous star formation rates .
The only non-quasar host galaxy , with modest star formation rate , for which dust extinction has been tentatively detected at these early cosmic epochs , is the host of gamma ray burst GRB050904 at z = 6.3 .
However , the claim of dust extinction for this GRB has been debated in the past .
We suggest that the discrepant results occur primarily because most of previous studies have not simultaneously investigated the X-ray to near-IR spectral energy distribution of this GRB .
The difficulty with this burst is that the X-ray afterglow is dominated by strong flares at early times and is poorly monitored at late times .
In addition , the Z band photometry , which is the most sensitive to dust extinction , has been found to be affected by strong systematics .
In this paper we carefully re-analyze the Swift/XRT afterglow observations of this GRB , using extensive past studies of X-ray flare properties when computing the X-ray afterglow flux level and exploiting the recent reanalysis of the optical ( UV rest frame ) data of the same GRB .
We extract the X-ray to optical/near-IR afterglow SED for the three epochs where the best spectral coverage is available : 0.47 , 1.25 , and 3.4 days after the trigger .
A spectral power-law model has been fitted to the extracted SEDs .
We discuss that no spectral breaks or chromatic temporal breaks are expected in the epochs of interest .
To fit any UV rest-frame dust absorption , we tested the Small Magellanic Cloud ( SMC ) extinction curve , the mean extinction curve ( MEC ) found for a sample of QSO at z > 4 and its corresponding attenuation curve , as well as a starburst attenuation curve , and the extinction curve consistent with a supernova dust origin ( SN-type ) .
The SMC extinction curve and the SN-type one provide good fit to the data at all epochs , with an average amount of dust absorption at \lambda _ { rest } = 3000 ~ { } \AA of A _ { 3000 } = 0.25 \pm 0.07 mag .
These results indicate that the primeval galaxy at z = 6.3 hosting this GRB has already enriched its ISM with dust .