Upper: 2D histogram of the correlation between the total gas column density, N_{H γ}, measured with the 0.4–100 GeV interstellar γ rays, and the dust optical depth at 353 GHz, convolved with the LAT response for an interstellar spectrum. The maps were sampled at a 0.̊375 resolution. Lower: evolution of the dust opacity in N_H bins. The error bars give the standard errors of the means and the grey band gives the standard deviation of the opacities in each bin.