Properties of spiral arms in the N-body simulation. (a): unsharp masking of stellar density in the galaxy (X,Y) plane, showing a density perturbation of the tightly wound spiral arm. (b): unsharp-masked stellar density distribution in guiding (X_g,Y_g) coordinates. (c): density distribution of stars in (X,Y), weighted by the positive density perturbation at their corresponding (X_g,Y_g) coordinates. (d): background map is a zoom-in of (b); coloured lines indicate the ±200 pc wide radial selection regions along the peaks of the distribution in this map. (e): distributions of galactocentric radii for all stars in two (X_g,Y_g) selection regions (the second overdensity is shown in red, and the fourth overdensity is plotted in purple as dashed lines). Solid lines correspond to the subset of stars that are located close to their guiding radii, |R − R_g|< 0.25 kpc. (f): background scatter plot shows the (X,Y) positions of all particles in the second (red) (X_g,Y_g) overdensity region, where the white contours correspond to 0.8, 0.5, 0.1, and 0.01 density levels. Filled circles show the locations of density maxima as illustrated in (panel e) for all overdensities marked in (panel d). (g): correspondence between the density perturbation of the spiral arm (zoom-in from panel a) and the location of such density maxima corresponding to all overdensities selected in (X_g,Y_g) (panel d). (h): background map represents the fraction of stars in (X,Y) coordinates near their guiding centres (dynamically cold population with |R − R_g|< 0.25), and the filled circles show the location of the same density maxima as in (panels f and g).