The Asymmetric Thick Disk: A Star-Count and Kinematic Analysis. II. The Kinematics

We report a kinematic signature associated with the observed asymmetry in thedistribution of thick disk/inner halo stars interior to the Solar circledescribed in Paper I. In that paper we found a statistically significant excess(20% to 25 %) of stars in quadrant I (l ~ 20 deg to 55 deg) both above andbelow the plane (b ~ +/- 25 deg to +/- 45 deg) compared to the complementaryregion in quadrant IV. We have measured Doppler velocities for 741 stars,selected according to the same magnitude and color criteria, in the directionof the asymmetry and in the corresponding fields in quadrant IV. We have alsodetermined spectral types and metallicities measured from the same spectra. Wenot only find an asymmetric distribution in the V_LSR velocities for the starsin the two regions, but the angular rate of rotation, w, for the stars inquadrant I reveals a slower effective rotation rate compared to thecorresponding quadrant IV stars. We use our [Fe/H] measurements to separate thestars into the three primary population groups, halo, thick disk, and disk, andconclude that it is primarily the thick disk stars that show the slowerrotation in quadrant I. A solution for the radial, tangential and verticalcomponents of the V_LSR velocities, reveals a significant lag of ~ 80 to 90km/s in the direction of Galactic rotation for the thick disk stars in quadrantI, while in quadrant IV, the same population has only a ~ 20 km/s lag. Theresults reported here support a rotational lag among the thick disk stars dueto a gravitational interaction with the bar as the most likely explanation forthe asymmetry in both the star counts and the kinematics. The affected thickdisk stars, however, may be associated with the recently discovered Canis Majordebris stream or a similar merger event (abridged).Comment: Accepted for publication in the Astronomical Journa