Vertical kinematics of the young Galactic clusters

ABSTRACT The young disc vertical phase is paramount in our understanding of Galaxy evolution. Analysing the vertical kinematics at different Galactic regions provides important information about the space–time variations of the Galactic potential. The vertical phase snail shell structure identified in Gaia Data Release 2 includes a wide age range. However, the structure of the $V_Z \ \mathrm{ versus} \ Z$ diagram appears linear when the analysis is limited to studying objects younger than 30 Ma. Based on the vertical velocity and height-over-disc maps obtained for a sample of young open clusters, this method also allows the matter density in the solar neighbourhood to be estimated using a completely different approach than previously found in the literature. We use two different catalogues of star clusters to confirm the previous result and study new age ranges. The linear pattern between $V_Z$ and Z shows different slopes, $\partial V_Z/\partial Z$, for various age groups. The results fit a simple model (harmonic oscillator) of in-plane decoupled vertical dynamics up to a certain age limit, corresponding to $\sim$30 Ma. This work also analyses the relationship between the local volumetric density of matter ($\rho _0$) and the disc vertical kinematics for different age ranges, all below 50 Ma. The best estimates of the effective volumetric mass density in the solar neighbourhood, 0.09–0.15 M$_\odot$ pc${}^{-3}$, agree with those given by other authors, assessing the reliability of the proposed dynamical model. These values are a minorant of the actual matter density in the region.