Histograms of the A_Ks values per pixel in the extinction maps. The purple, green, orange, and red histograms correspond to the spiral arms. The grey histogram was obtained using the extinction map in Nogueras-Lara et al. (2020a), corresponding to the central region of the NSD. Given the smaller area of the NSD extinction map, the spiral arm histograms were properly scaled. Filled coloured contours show Gaussian fits, whose mean and standard deviation are indicated in the figure.