Mapping the Asymmetric Thick Disk: III. The Kinematics and Interaction with the Galactic Bar

TL;DR

This study maps the kinematic asymmetries of the Thick Disk in the inner Galaxy, revealing a rotational lag associated with the Galactic bar, suggesting a gravitational interaction influencing stellar motions.

Contribution

It provides detailed kinematic measurements of Thick Disk stars in the inner Galaxy, demonstrating a link between the Galactic bar and stellar motion asymmetries not previously characterized.

Findings

Thick Disk stars in Q1 lag by 60-70 km/s in rotation.

Metal-Weak Thick Disk stars lag by about 100 km/s.

Rotational lag decreases when lines of sight pass over the bar.

Abstract

In the first two papers of this series, Larsen et al (2010a,b) describe our faint CCD survey in the inner Galaxy and map the over-density of Thick Disk stars in Quadrant I (Q1) to 5 kpc or more along the line of sight. The regions showing the strongest excess are above the density contours of the bar in the Galactic disk. In this third paper on the asymmetric Thick Disk, we report on radial velocities and derived metallicity parameters for over 4000 stars in Q1, above and below the plane and in Q4 above the plane. We confirm the corresponding kinematic asymmetry first reported by Parker et al. (2004), extended to greater distances and with more spatial coverage. The Thick Disk stars in Q1 have a rotational lag of 60 -- 70 km/s relative to circular rotation, and the Metal-Weak Thick Disk stars have an even greater lag of 100 km/s. Both lag their corresponding populations in Q4 by approximately 30 km/s. Interestingly, the Disk stars in Q1 also appear to participate in the rotational lag by about 30 km/s. The enhanced rotational lag for the Thick Disk in Q1 extends to 4 kpc or more from the Sun. At 3 to 4 kpc, our sight lines extend above the density contours on the near side of the bar, and as our lines of sight pass directly over the bar the rotational lag appears to decrease. This is consistent with a "gravitational wake" induced by the rotating bar in the Disk which would trap and pile up stars behind it. We conclude that a dynamical interaction with the stellar bar is the most probable explanation for the observed kinematic and spatial asymmetries.