Left: dust optical depth at Hα wavelength τ_Hα overlaid with contours of the Hα emission from NGC 6946. Contour levels are 1, 3, 6, 15, 30, 40 μerg  s^-1 cm^-2 sr^-1. The bar at the right shows the values of τ_Hα. Right: radiation field U map (mass-weighted mean starlight heating intensity) of NGC 6946 in units of radiation field in solar neighborhood U_⊙ (Aniano et al. 2012), indicated by the bar at the right of the image. The beam area is shown in the lower left corners.