Left panel: map of the ratios of hydrogen column densities derived from interstellar γ rays in the “γ + E(B − V)” and “γ + τ₃₅₃” analyses, thewhite contours marking the positions of γ-ray point sources. Middle and right panels: residual γ-ray maps (photon counts minus best-fit model, in sigma units) in the 0.4–100 GeV band for the “γ + E(B − V)” (middle) and “γ + τ₃₅₃” (right) analyses, for an HI spin temperature of 400 K. The black contours outline the CO clouds at the 1 K km s⁻¹ level.