Dust properties at z=6.3 in the host galaxy of GRB 050904

We investigate the dust extinction properties in the host galaxy of GRB 050904 at z = 6.29 by analyzing simultaneous broadband observations of the optical and UV afterglow at three different epochs. We show that the peculiar afterglow spectral energy distribution ( SED) observed at 0.5 days and at 1 day after the burst ( 1.6 and 3 hr rest frame) cannot be explained by dust reddening with any of the extinction curves observed at low redshift. Yet, the extinction curve recently inferred for the most distant BAL QSO z = 6.2 nicely reproduces the SED of GRB 050904 at both epochs. Our result provides an additional, independent indication that the properties of dust evolve beyond z similar to 6. We discuss the implications of this finding within the context of the dust production mechanisms through the cosmic ages.