Calibrating Milky Way dust extinction using cosmological sources

We constrain the light extinction properties of Milky Way dust. We investigated the correlations between dust column density as inferred from infrared data and the observed colours of celestial objects at cosmological distances with low levels of colour dispersion. Results derived using colours of quasars, brightest central galaxies, and luminous red galaxies are broadly consistent, indicating a proportionality constant between the reddening E(B - V) = A(B) - A(V) and the dust column density D-T (given in units of MJy/sr) of p = E(B - V)/D-T similar to 0.02 and a reddening parameter R-V = Lambda(V)/E(B - V) similar to 3 with fractional uncertainties of approximately 10%. The data do not provide any evidence for spatial variations in the dust properties, except for a possible hint of scatter in the dust extinction properties at the longest optical wavelengths.