Geometry and Kinematics of a Dancing Milky Way: Unveiling the Precession and Inclination Variation across the Galactic Plane via Open Clusters

TL;DR

This study uses Gaia data to analyze the Milky Way's disk geometry, revealing its warp, precession, and inclination variations, and providing new measurements of these dynamic features with implications for Galactic structure understanding.

Contribution

It offers the first detailed analysis of Galactic disk precession and inclination variation using open clusters, highlighting the non-stationary mid-plane and dynamic warp of the Milky Way.

Findings

Galactic disk inclination increases outward from the center.

The Milky Way's mid-plane is not fixed but shifts over time.

Precession rate of the Galactic disk is approximately 6.8 km/s/kpc.

Abstract

This Letter presents a study of the geometry and motion of the Galactic disk using open clusters in the Gaia era. The findings suggest that the inclination of the Galactic disk increases gradually from the inner to the outer disk, with a shift in orientation at the Galactocentric radius of approximately 5 to 7 kpc. Furthermore, this study brings forth the revelation that the mid-plane of the Milky Way may not possess a stationary or fixed position. A plausible explanation is that the inclined orbits of celestial bodies within our Galaxy exhibit a consistent pattern of elliptical shapes, deviating from perfect circularity; however, more observations are needed to confirm this. An analysis of the vertical motion along the Galactocentric radius reveals that the disk has warped with precession, and that the line-of-nodes shifts at different radii, aligning with the results from the classical Cepheids. Although there is uncertainty for precession/peculiar motion in Solar orbit, after considering the uncertainty, the study derives a median value of precession rate = 6.8 km/s/kpc in the Galaxy. This value for the derived precession in the outer disk is lower than those in the literature due to the systematic motion in Solar orbit (inclination angle = 0.6 deg). The study also finds that the inclinational variation of the disk is significant and can cause systematic motion, with the inclinational variation rate decreasing along the Galactic radius with a slope of -8.9 uas/yr/kpc. Moreover, the derived inclinational variation rate in Solar orbit is 59.1+-11.2(sample)+-7.7(VZsun) uas/yr, which makes it observable for high precision astrometry. The all-sky open cluster catalog based on Gaia DR3 and Galactic precession/inclinational variation fits as well as Python code related to these fits are available at https://nadc.china-vo.org/res/r101288/