Comparison between density distribution of OB stars, the spiral-arm model from Reid et al. (2019), and the radial velocity asymmetries. In the left panel and central panels, the solid black contours represent the 0 and 0.5 density-contrast levels. In the right panel, the spiral arms are named following Reid et al. (2019): Outer Arm; Perseus Arm; Local Arm; Sagittarius-Carina Arm; Scutum-Centaurus Arm. In the three panels, the yellow star indicates the Sun’s position in X, Y=−8.2, 0 kpc, and the Galactic centre is located in X, Y = 0, 0 kpc. The dashed lines represent circle radii of 2, 4, and 6 kpc. The solid lines indicate constant Galactic longitudes.