The asymmetric thick disk: A star-count and kinematic analysis. II. The kinematics

We report a kinematic signature associated with the observed asymmetry in the distribution of thick-disk/inner halo stars interior to the solar circle described in Paper I. In that paper, we found a statistically significant excess (20%-25%) of stars in quadrant I (l ∼ 20°-55°) both above and below the plane (b ∼ ± 25° to ± 45°) compared with the complementary region in quadrant IV. We have measured Doppler velocities for 741 stars, selected according to the same magnitude and color criteria, in the direction of the asymmetry and in the corresponding fields in quadrant IV. We have also determined spectral types and metallicities measured from the same spectra. We not only find an asymmetric distribution in the V _LSR velocities for the stars in the two regions, but the angular rate of rotation (ω) for the stars in quadrant I reveals a slower effective rotation rate compared with the corresponding quadrant IV stars. The results for V _LSR and ω also show an interesting dependence on Galactic longitude that is most pronounced in quadrant I. We use our [Fe/H] measurements to separate the stars into the three primary population groups, halo, thick disk, and disk, and conclude that it is primarily the thick-disk stars that show the slower rotation in quadrant I. These stars are also responsible for the observed variation of V _LSR and ω with Galactic longitude. A solution for the radial, tangential, and vertical components of the V _LSR velocities reveals a significant lag of ≈80 to 90 km s ^-1 in the direction of Galactic rotation for the thick-disk stars in quadrant I, while in quadrant IV, the same population has only a ∼20 km s ^-1 lag confirming the kinematic asymmetry between the two directions. In Paper I, we concluded that the asymmetry in the star counts could be best explained by either a triaxial thick disk or an interaction between the bar in the disk and the thick-disk/inner halo stars. The results reported here support a rotational lag among the thick-disk stars due to a gravitational interaction with the bar as the most likely explanation for the asymmetry in both the star counts and the kinematics. The affected thick-disk stars, however, may be associated with the recently discovered Canis Major debris stream or a similar merger event.