The Galactic Disc in Action Space as seen by Gaia DR2

Wilma H. Trick; Johanna Coronado; Hans-Walter Rix

doi:10.1093/mnras/stz209

paper

The Galactic Disc in Action Space as seen by Gaia DR2

arXiv:1805.03653 · doi:10.1093/mnras/stz209

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 $(J_{R}, J_{ϕ} = L_{z}, J_{z})$ of $\sim 3.5$ million stars within $1.5 kpc$ of the Sun, for which precise actions can be calculated from Gaia DR2 alone. This distribution $n (J_{R}, L_{z})$ reveals a remarkable amount of sub-structure. The known moving groups in the $(U, V)$ -plane of the Solar neighborhood correspond to overdensities in $(J_{R}, L_{z})$ , as expected. But $n (J_{R}, L_{z})$ also exhibits a wealth of density clumps and ridges that extend towards higher $J_{R}$ . These $n (J_{R}, L_{z})$ features are most prominent among orbits that stay close to the Galactic plane and remain consistently visible out to $\sim 1.5 kpc$ , as opposed to the sub-structure in velocity space. Some of these $n (J_{R}, L_{z})$ ridges resemble features expected from rapid orbit diffusion along particular ( $J_{R}, L_{z}$ )-directions in the presence of various resonances. Several of these $n (J_{R}, L_{z})$ structures show a dramatic imbalance of stars moving in or out, suggesting that stars are not phase-mixed along orbits or on resonant orbits. Orbital action and angle space of stars in Gaia DR2 is therefore highly structured over kpc-scales, and appears to be very informative for modeling studies of non-axisymmetric structure and resonances in the Galactic disc.

arXiv comment17 pages, 9 figures, 1 table, accepted by MNRAS

The Galactic Disc in Action Space as seen by Gaia DR2

Abstract

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