Left panel: Median radial velocity, Ṽ_R, for the fiducial non-axisymmetric model in the disk plane (x,y) defined on [−13,13] kpc × [−13, 13] kpc binned with (250 pc)² bins. Right panel: Adaptation of panel B in figure 1 of Poggio et al. (2021) with over-densities of young upper main sequence stars in red tracing the position of the arms segments. Both panels display the bar position of our fiducial model with dashed black lines, the major m = 2 (black) and minor m = 3 (gray) spiral locations of our fiducial model as continuous and dotted black and gray lines, respectively. The continuous lines indicate the maximum overdensity of the model spirals as a function of radius from their (negative) potential minimum, down to the cutoff radius. The dotted line then traces an arc of a circle at each cutoff radius, until the point where the spiral potential reaches zero. In both panels, the cross represents the Sun’s position. The overdensity maps of young stars following Poggio et al. (2021) were generated from https://github.com/epoggio/Spiral_arms_EDR3.git.