2021MNRAS.506..523V

We observe the density wave's angular pattern speed _p to be near 12-17 km s^–1 kpc^–1, by the separation between a typical optical H II region (from the spiral arm's dust lane) and using a H II-evolution time model T_H II_ to yield its relative speed, and independently by the separation between a typical radio maser (from the spiral arm's dust lane) with a maser model. The assumption of a fixed circular rotational speed of the gas and stars with galactic radius is employed (neglecting spiral perturbation at mid-radii, nuclear bar influence at small radii, and tidal effects at large radii).