The tilt of the velocity ellipsoid of different Galactic disk populations

Abstract

The tilt of the velocity ellipsoid is a helpful tracer of the gravitational potential of the Milky Way. In this paper, we use nearly 140,000 RC stars selected from the LAMOST and Gaia to make a detailed analysis of the tilt of the velocity ellipsoid for various populations, as defined by the stellar ages and chemical information, within 4.5 $\leq$ $R$ $\leq$ 15.0 kpc and $|Z|$ $\leq$ 3.0 kpc. The tilt angles of the velocity ellipsoids of the RC sample stars are accurately described as $\alpha$ = $\alpha_{0}$ $\mathrm{arctan}$ ($Z$/$R$) with $\alpha_{0}$ = (0.68 $\pm$ 0.05). This indicates the alignment of velocity ellipsoids is between cylindrical and spherical, implying that any deviation from the spherical alignment of the velocity ellipsoids may be caused by the gravitational potential of the baryonic disk. The results of various populations suggest that the $\alpha_{0}$ displays an age and population dependence, with the thin and thick disks respectively values $\alpha_{0}$ = (0.72 $\pm$ 0.08) and $\alpha_{0}$ = (0.64 $\pm$ 0.07), and the $\alpha_{0}$ displays a decreasing trend with age (and [$\alpha$/Fe]) increases, meaning that the velocity ellipsoids of the kinematically relaxed stars are mainly dominated by the gravitational potential of the baryonic disk. We determine the $\alpha_{0} - R$ for various populations, finding that the $\alpha_{0}$ displays oscillations with $R$ for all the different populations. The oscillations in $\alpha_{0}$ appear in both kinematically hot and cold populations, indicating that resonances with the Galactic bar are the most likely origin for these oscillations.