2018ApJ...863...52V

The radial distance to the corotation radius R_coro (where the angular speed of the gas and stars, Ω_gas, in orbit around the Galactic center (GC) is equal to the angular speed of the spiral arm pattern, Ω_sp) has often been predicted (at various places), but not measured with high precision. Here we test the locations of masers with respect to the Perseus arm (Table 1). Our analysis of the masers and H II regions near the Perseus arm (mostly located on the inner arm side, by about 0.4 ± 0.1 kpc from the cold CO mid-arm) shows that the corotation R_coro must be >10.8 kpc from the GC (Figure 1). This implies that the angular rotation speed of the spiral pattern Ω_sp< 21.3 km s^–1 kpc^–1. Another test in Galactic quadrant II shows that the radial velocity of the masers is generally more negative than that of the CO mid-arm (Figure 2), indicating a deceleration with respect to the CO mid-arm, by about 9 ± 3 km s^–1. This implies that Ω_sp < 20.7 km s^–1 kpc^–1, and thus R_coro > 11.1 kpc. Finally, comparing our results with other published results (Table 2), we find a statistical mean corotation radius R_coro predicted to be near 12 ± 1 kpc from the GC (beyond the Perseus arm; before the Cygnus arm), and a mean angular spiral pattern speed, Ω_sp, predicted to be near 19 ± 2 km s^–1 kpc^–1.