Environmental mask of M 51. Based on a visual inspection of optical HST data and ¹²CO (2–1) emission, we generated the following environmental mask. The pixels are colour-coded by their associated region. This includes the centre (bright orange), interarm (dark orange), spiral arm (magenta), and disc (purple). The centre includes data points within a 45 arcsec aperture. White contours indicate ¹²CO(2–1) emission at 7, 10, 15, and 30 K km s⁻¹.