The Galactic Disc in Action Space as seen by Gaia DR2
Wilma H. Trick, Johanna Coronado, Hans-Walter Rix

TL;DR
This study uses Gaia DR2 data to analyze the detailed structure of stellar orbits in the Galactic disc, revealing complex sub-structures and resonances in action space that inform models of Galactic dynamics.
Contribution
It provides the first large-scale analysis of orbital actions for millions of stars, uncovering detailed sub-structures and resonances in the Galactic disc beyond the Solar neighborhood.
Findings
Identification of sub-structures in action space correlating with known moving groups
Detection of density ridges indicating orbital diffusion and resonances
Evidence of stars not being phase-mixed along their orbits
Abstract
The quality and quantity of 6D stellar position-velocity measurements in the second Gaia data release (DR2) allows to study small-scale structure in the orbit distribution of the Galactic disc beyond the immediate Solar neighborhood. We investigate the distribution of orbital actions of million stars within of the Sun, for which precise actions can be calculated from Gaia DR2 alone. This distribution reveals a remarkable amount of sub-structure. The known moving groups in the -plane of the Solar neighborhood correspond to overdensities in , as expected. But also exhibits a wealth of density clumps and ridges that extend towards higher . These features are most prominent among orbits that stay close to the Galactic plane and remain consistently visible out to…
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