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last change 2008 March 26, R. Arlt
Galactic dynamics
Galactic kinematics with RAVE data
The distribution of stars towards the Galactic poles

The knowledge of the kinematical structure of the Milky Way disk is an important constraint on how the Galaxy has been formed. While there is common consensus that the disk of the Galaxy consists of two components - the thin and the thick disk - the structure of the thick disk is not well known. Scale heights range from 700 pc (Cabrera-Lavers et al. 2005) to 1500 pc (Gilmore & Reid 1983). Also the relation between the thin and the disk is not clear. The stellar density distribution shows a change of slope in the logarithm of the vertical distribution. But, is there a continuous transition or a clear separation?

The combination of the photometric and kinematic informations can help to answer this question in identifying more precisely the thin and thick disk. In the past, such studies were limited to some specific directions. Now, all sky survey like 2MASS for photometry, the UCAC2 for proper motions and RAVE for radial velocities give global vision of the Galaxy. To describe these new and accurate data, we have developed a self-consistent dynamical model. The principal challenge of this models is the number of free parameters. Our model is not limited to two characteristic velocity dispersions one for thin disk and the other for the thick disk. Instead, a range of possible velocity dispersions has been used. The Galactic disk is, in this way, decomposed in kinematic components.

It appears that there is a clear kinematic discontinuity in the kinematical decomposition (Figure 1). There are two mains features with vertical velocity dispersion sigma_W = [10-25] km/s and sigma_W = [30-45] km/s that can be recognized as the thin and thick disk. This proves that the thin and thick disk are indeed two distinct components of the Galactic disk. With this decomposition, a scale height of 220 pc for the thin disk and 1030 pc for the thick disk has been measured (Figure 2). The kinematic identification of the thin and thick disk links those structures to physical process of the formation of the disk.

Moreover, this kinematical discontinuity between the thin and thick disk put some constrains on the formation process of the Galactic disk. Scenarios with a thick disk which would be a heated thin disk by molecular clouds or spiral arms can be rejected. Other scenarios, e.g., in which the thick disk is created by accretion events are compatible with our findings.


Veltz, L., Bienaymé, O., Freeman, K.C., Binney, J., Bland-Hawthorn, J., Gibson, B.K., Gilmore, G., Grebel, E.K., Helmi, A., Munari, U., Navarro, J.F., Parker, Q.A., Seabroke, G.M., Siebert, A., Steinmetz, M., Watson, F.G., Williams, M., Wyse, R.F.G., Zwitter, T.: Galactic kinematics with RAVE data: I. The distribution of stars towards the Galactic poles. Astronomy and Astrophysics 480 (2008), 753.

Dr. Lionel Veltz
Astrophysikalisches Institut Potsdam
An der Sternwarte 16
D-14482 Potsdam
(0331) 7499 531


Figure 1: The local sigma_W kinematic distribution function: a thin disk component (sigam_W = [10-25] km/s) and a thick disk (sigam_W = [30-45] km/s).

Figure 2: Model of the vertical stellar density rho(z) (dashed line) and its thin and thick disk decomposition (respectively thin and thick lines).




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