Top: 2D histograms of the correlations between the A_V data and the dust optical depth at 353 GHz (Planck Collaboration XI 2014) or the total gas column density, N_{H γ}, measured with 0.4–100 GeV interstellar γ rays (Remy et al. 2017) (bottom). All the maps are compared at a common angular resolution in 0. °25 × 0. °25 bins. The orange line in the top plots corresponds to a ratio A_V∕τ₃₅₃ = 4.62 × 10⁴ mag (Planck Collaboration XI 2014). The green line in the bottom plots corresponds to a ratio A_V ∕N_H = 5.34 × 10⁻²² mag cm² (Bohlin et al. 1978). In each panel the magenta line corresponds to the best fit of a linear regression. The blue and red areas correspond to the pixel selections used to generate the contours in Fig. A.1.