Dust optical depth measured at 353 GHz from the Planck and IRAS data and displayed on a 10′.2×7′.5 grid at the angular resolution of Planck (top panel) and of Fermi-LAT (middle). The bottom panel shows the γ-ray photon counts produced by CR interactions with gas and recorded by Fermi-LAT in 0°25×0°25 bins in the 0.16–63 GeV energy band (other emissions of non-gaseous origin are subtracted). Three zones are excluded from the analysis region: the SMC on the left, LMC in the middle, and the edge of the Gum nebula in the upper right. The Reticulum cloud is the long filament crossing the region below the LMC.