The Galactic Disc in Action Space as seen by Gaia DR2. (arXiv:1805.03653v2 [astro-ph.GA] UPDATED)

The Galactic Disc in Action Space as seen by Gaia DR2. (arXiv:1805.03653v2 [astro-ph.GA] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Trick_W/0/1/0/all/0/1">Wilma H. Trick</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Coronado_J/0/1/0/all/0/1">Johanna Coronado</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rix_H/0/1/0/all/0/1">Hans-Walter Rix</a>

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
$(J_R,J_phi=L_z,J_z)$ of $sim 3.5$ million stars within $1.5~text{kpc}$ 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 $sim1.5~text{kpc}$, 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.

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
$(J_R,J_phi=L_z,J_z)$ of $sim 3.5$ million stars within $1.5~text{kpc}$ 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 $sim1.5~text{kpc}$, 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.

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