Interstellar extinction curve variations towards the inner Milky Way: A challenge to observational cosmology

We investigate interstellar extinction curve variations towards ~4 deg² of the inner MilkyWay in VIJKs photometry from the OGLE-III (third phase of the Optical Gravitational Lensing Experiment) and VVV (VISTA Variables in the Via Lactea) surveys, with supporting evidence from diffuse interstellar bands and F435W, F625W photometry. We obtain independent measurements towards ~2000 sightlines of A_I, E(V - I), E(I - J) and E(J - K_s), with median precision and accuracy of 2 per cent. We find that the variations in the extinction ratios A_I/E(V - I), E(I - J)/E(V - I) and E(J - K_s)/E(V - I) are large (exceeding 20 per cent), significant and positively correlated, as expected. However, both the mean values and the trends in these extinction ratios are drastically shifted from the predictions of Cardelli and Fitzpatrick, regardless of how R_V is varied. Furthermore, we demonstrate that variations in the shape of the extinction curve have at least two degrees of freedom, and not one (e.g. R_V), which we confirm with a principal component analysis. We derive a median value of A_V/A_Ks = 13.44, which is ~60 per cent higher than the 'standard' value. We show that theWesenheit magnitude W_I = I - 1.61(I - J) is relatively impervious to extinction curve variations. Given that these extinction curves are linchpins of observational cosmology, and that it is generally assumed that R_V variations correctly capture variations in the extinction curve, we argue that systematic errors in the distance ladder from studies of Type Ia supernovae and Cepheids may have been underestimated. Moreover, the reddening maps from the Planck experiment are shown to systematically overestimate dust extinction by ~100 per cent and lack sensitivity to extinction curve variations.