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We have observed a snapshot of our N-body/Smoothed Particle Hydrodynamicssimulation of a Milky Way-sized barred spiral galaxy in a similar way to how wecan observe the Milky Way. The simulated galaxy shows a co-rotating spiral arm,i.e. the spiral arm rotates with the same speed as the circular speed. Weobserved the rotation and radial velocities of the gas and stars as a functionof the distance from our assumed location of the observer at the three lines ofsight on the disc plane, (l, b) = (90, 0), (120, 0) and (150,0) deg. We findthat the stars tend to rotate slower (faster) behind (at the front of) thespiral arm and move outward (inward), because of the radial migration. However,because of their epicycle motion, we see a variation of rotation and radialvelocities around the spiral arm. On the other hand, the cold gas componentshows a clearer trend of rotating slower (faster) and moving outward (inward)behind (at the front of) the spiral arm, because of the radial migration. Wehave compared the results with the velocity of the maser sources from Reid etal. (2014), and find that the observational data show a similar trend in therotation velocity around the expected position of the spiral arm at l = 120deg. We also compared the distribution of the radial velocity from the localstandard of the rest, V_LSR, with the APOGEE data at l = 90 deg as an example.

Gas and Stellar Motions and Observational Signatures of Co-Rotating Spiral Arms