Milky Way dust extinction measured with QSOs

We investigate reddening by Milky Way dust in the low-extinction regime of E(B - V) < 0.15. Using over 50 000 QSOs at 0.5 < z < 2.5 from the Sloan Digital Sky Survey (SDSS) data release 7 (DR7) QSO Catalogue, we probe the residual SDSS colours after dereddening and correcting for the known spectroscopic redshifts.We find that the extinction vector of Schlafly & Finkbeiner is a better fit to the data than that used by Schlegel et al. (SFD). There is evidence for a non-linearity in the SFD reddening map, which is similarly present in the V1.2 map of the Planck Collaboration. This non-linearity is similarly seen when galaxies or stars are used as probes of the SFD map.