eRASS:4 exposure-corrected intensity sky map; the same as the one displayed on the left panel of Fig. 1. Left panel: Canadian Galactic Plane Survey (CGPS) data at 1.42 GHz (Taylor et al. 2003) are overlaid as contours. Right panel: Optical Hα data obtained from the full-sky Hα map of 6′ FWHM resolution (Finkbeiner 2003) are overlaid as contours. For both panels, three level contours are used from thin, dashed, white-on-thick-solid-black, to thin, solid black on thick solid white, and solid white to represent fainter-to-brighter emission regions.