The 3D Velocity Structure of the Thick Disk from SPM4 and RAVE DR2

TL;DR

This study maps the 3D velocity structure of the Milky Way's thick disk using a large star sample, revealing velocity gradients, dispersions, and orbital characteristics that inform galaxy formation models.

Contribution

It provides new detailed measurements of thick disk kinematics and compares observed orbital eccentricities with formation models, favoring merger scenarios.

Findings

Velocity gradient of -25.2 km/s/kpc in the thick disk.

Velocity dispersions at 1 kpc height: (70.4, 48.0, 36.2) km/s.

Stars beyond 0.7 kpc show a net inward radial velocity of 13 km/s.

Abstract

We analyze the 3D kinematics of a sample of $\sim 4400$ red clump stars ranging between 5 and 10 kpc from the Galactic center and up to 3 kpc from the Galactic plane. This sample is representative for the metal-rich ([Fe/H] = -0.6 to 0.5) thick disk. Absolute proper motions are from the fourth release of the Southern Proper Motion Program, and radial velocities from the second release of the Radial Velocity Experiment. The derived kinematical properties of the thick disk include: the rotational velocity gradient $\partial V_{\theta} / \partial z = -25.2 \pm 2.1$ km s$^{-1}$ kpc$^{-1}$, velocity dispersions $(\sigma_{V_R}, \sigma_{V_{\theta}}, \sigma_{V_z})|_{z=1} = (70.4, 48.0, 36.2) \pm(4.1,8.3,4.0)$ km s$^{-1}$, and velocity-ellipsoid tilt angle $\alpha_{Rz} = 8.6\arcdeg \pm 1.8 \arcdeg$. Our dynamical estimate of the thin-disk scale length is $R_{thin} = 2.0 \pm 0.4$ kpc and the thick-disk scale height is $z_{thick} = 0.7 \pm 0.1$ kpc. The observed orbital eccentricity distribution compared with those from four different models of the formation of the thick disk from Sales et al. favor the gas-rich merger model and the minor merger heating model. Interestingly, when referred to the currently accepted value of the LSR, stars more distant than 0.7 kpc from the Sun show a net average radial velocity of $13 \pm3 $ km s$^{-1}$. This result is seen in previous kinematical studiesusing other tracers at distances larger than $\sim 1$ kpc. We suggest this motion reflects an inward perturbation of the locally-defined LSR induced by the spiral density wave.