Kinematical and chemical vertical structure of the Galactic thick disk I. Thick disk kinematics

TL;DR

This study analyzes the vertical kinematic and chemical structure of the Galactic thick disk using 412 red giants, revealing velocity gradients, possible substructure, and implications for the disk's formation and orientation.

Contribution

It provides detailed measurements of thick disk kinematics up to 4.5 kpc from the plane, including velocity gradients and tilt angles, and discusses their implications for Galactic formation models.

Findings

Rotational velocity decreases with height, fitting a power-law model.

Velocity dispersions increase with height, with small vertical gradients.

Possible kinematic substructure detected between 2.5 and 3.5 kpc.

Abstract

The variation of the kinematical properties of the Galactic thick disk with Galactic height Z are studied by means of 412 red giants observed in the direction of the south Galactic pole up to 4.5 kpc from the plane. We confirm the non-null mean radial motion toward the Galactic anticenter found by other authors, but we find that it changes sign at |Z|=3 kpc, and the proposed inward motion of the LSR alone cannot explain these observations. The rotational velocity decreases with |Z| by -30 km/s/kpc, but the data are better represented by a power-law with index 1.25, similar to that proposed from the analysis of SDSS data. All the velocity dispersions increase with |Z|, but the vertical gradients are small. The dispersions grow proportionally, with no significant variation of the anisotropy. The ratio sigma_U/sigma_W=2 suggests that the thick disk could have formed from a low-latitude merging event. The vertex deviation increases with Galactic height, reaching ~20 degrees at |Z|=3.5 kpc. The tilt angle also increases, and the orientation of the ellipsoid in the radial-vertical plane is constantly intermediate between the alignment with the cylindrical and the spherical coordinate systems. The tilt angle at |Z|=2 kpc coincides with the expectations of MOND, but an extension of the calculations to higher |Z| is required to perform a conclusive test. Finally, between 2.5 and 3.5 kpc we detect deviations from the linear trend of many kinematical quantities, suggesting that some kinematical substructure could be present.