The signatures of the outer Lindblad resonance and corotation of a large Galactic bar in local velocity space. (arXiv:1812.04151v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Monari_G/0/1/0/all/0/1">Giacomo Monari</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Famaey_B/0/1/0/all/0/1">Benoit Famaey</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Siebert_A/0/1/0/all/0/1">Arnaud Siebert</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wegg_C/0/1/0/all/0/1">Christopher Wegg</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gerhard_O/0/1/0/all/0/1">Ortwin Gerhard</a>

The second data release of the Gaia mission has revealed a very rich
structure in local velocity space, which is related to resonances with multiple
non-axisymmetric patterns and possibly to incomplete phase-mixing. In terms of
in-plane motions, this rich structure is also seen as ridges in the actions of
the axisymmetric background potential of the Galaxy. We have recently developed
a method to capture the behaviour of the stellar phase-space distribution
function at a resonance, by re-expressing it in terms of a new set of canonical
actions and angles variables valid in the resonant region. At least some of the
action space ridges seen in Gaia data must obviously be linked to resonances
with the central bar of the Galaxy. However, the structure and pattern speed of
the bar have been heavily debated in recent years. Here, by properly treating
the distribution function at resonances, and by using a realistic model for the
Galactic bar, we confirm that orbits trapped at the corotation of a slowly
rotating large bar produce a structure akin to the Hercules moving group in
local velocity space (P’erez-Villegas et al. 2017). We additionally show that
a second prominent ridge in action space then corresponds to the 4:1 outer
resonance of the $m=4$ mode of such a bar, and that the velocity structure seen
as an arch at high azimuthal velocities in Gaia data can be related to its 2:1
outer Lindblad resonance.

The second data release of the Gaia mission has revealed a very rich
structure in local velocity space, which is related to resonances with multiple
non-axisymmetric patterns and possibly to incomplete phase-mixing. In terms of
in-plane motions, this rich structure is also seen as ridges in the actions of
the axisymmetric background potential of the Galaxy. We have recently developed
a method to capture the behaviour of the stellar phase-space distribution
function at a resonance, by re-expressing it in terms of a new set of canonical
actions and angles variables valid in the resonant region. At least some of the
action space ridges seen in Gaia data must obviously be linked to resonances
with the central bar of the Galaxy. However, the structure and pattern speed of
the bar have been heavily debated in recent years. Here, by properly treating
the distribution function at resonances, and by using a realistic model for the
Galactic bar, we confirm that orbits trapped at the corotation of a slowly
rotating large bar produce a structure akin to the Hercules moving group in
local velocity space (P’erez-Villegas et al. 2017). We additionally show that
a second prominent ridge in action space then corresponds to the 4:1 outer
resonance of the $m=4$ mode of such a bar, and that the velocity structure seen
as an arch at high azimuthal velocities in Gaia data can be related to its 2:1
outer Lindblad resonance.

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