Example of the environmental masks for NGC4321, overlaid on the Hα intensity map (Razza et al., in prep.). The width of the original spiral arm mask (white contours) from Querejeta et al. (2021) is adjusted by convolution with a Gaussian kernel (dashed pink contours) in order to visually encompass all the relevant Hα emission from the spiral structures. The red contour outlines the field-of-view of the ALMA CO(2-1) map, which limits the region in which our analysis can be performed. White stars mark the position of the identified emission peaks in the Hα map with the HEISENBERG code (see Section 3 and Kim et al. 2022).