  We have observed a snapshot of our N-body/Smoothed Particle Hydrodynamics
simulation of a Milky Way-sized barred spiral galaxy in a similar way to how we
can 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. We
observed the rotation and radial velocities of the gas and stars as a function
of the distance from our assumed location of the observer at the three lines of
sight on the disc plane, (l, b) = (90, 0), (120, 0) and (150,0) deg. We find
that the stars tend to rotate slower (faster) behind (at the front of) the
spiral arm and move outward (inward), because of the radial migration. However,
because of their epicycle motion, we see a variation of rotation and radial
velocities around the spiral arm. On the other hand, the cold gas component
shows a clearer trend of rotating slower (faster) and moving outward (inward)
behind (at the front of) the spiral arm, because of the radial migration. We
have compared the results with the velocity of the maser sources from Reid et
al. (2014), and find that the observational data show a similar trend in the
rotation velocity around the expected position of the spiral arm at l = 120
deg. We also compared the distribution of the radial velocity from the local
standard of the rest, V_LSR, with the APOGEE data at l = 90 deg as an example.
