H₂ column density maps (grey scale and black contour) derived from Herschel data with the ¹²CO (1–0) intensity maps overplotted as green contours. Top panel: range of Galactic longitude 219°–230°; the ¹²CO (1–0) emission is integrated over the velocity range 5–75 km s⁻¹; green contours are at 5, 25 and 50 K km s⁻¹. Bottom panel: range 230°–240°; the ¹²CO (1–0) emission is integrated over the velocity range 13–87 km s⁻¹; green contours are at 5, 25, and 50 K km s⁻¹. The intensity bar of the H₂ column density at the bottom is in units of cm⁻². The black contour corresponds to N(H₂) = 2.8 × 10²¹ cm⁻², equivalent to a visual extinction level A_V = 3 mag.