Radial velocities in the outermost disk toward the anticenter

We measure the mean Galactocentric radial component of the velocity of stars ($v_R$) in the disk at 8 kpc$<R<28$ kpc in the direction of the anticenter. For this, we use the Apache Point Galactic Evolution Experiment (APOGEE). Furthermore, we compare the result with HI maps along the same line of sight. We find an increase in positive (expansion) $v_R$ at $R\approx 9-13$ kpc, reaching a maximum of $\approx 6$ km/s, and a decrease at large values of $R$ reaching a negative (contraction) value of $\approx -10$ km/s for $R>17$ kpc. Negative velocities are also observed in 21 cm HI maps, possibly dominated by local gas emission. Among the possible dynamical causes for these non-zero $v_R$, factors such as the effect of the Galactic bar, streams, or mergers do not seem appropriate to explain our observations. An explanation might be the gravitational attraction of overdensities in a spiral arm. As a matter of fact, we see a change of regime from positive to negative velocities around $R\approx 15$ kpc, in the position where we cross the Outer spiral arm in the anticenter. The mass in spiral arms necessary to produce these velocities would be about 3\% of the mass of the disk, consistent with our knowledge of the spiral arms. Another scenario that we explore is a simple class of out-of-equilibrium systems in which radial motions are generally created by the monolithic collapse of isolated self-gravitating overdensities.