Identifying resonances of the Galactic bar in Gaia DR2: I. Clues from action space. (arXiv:1906.04786v3 [astro-ph.GA] UPDATED)

Identifying resonances of the Galactic bar in Gaia DR2: I. Clues from action space. (arXiv:1906.04786v3 [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:+Fragkoudi_F/0/1/0/all/0/1">Francesca Fragkoudi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hunt_J/0/1/0/all/0/1">Jason A. S. Hunt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mackereth_J/0/1/0/all/0/1">J. Ted Mackereth</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+White_S/0/1/0/all/0/1">Simon D. M. White</a>

Action space synthesizes the orbital information of stars and is well-suited
to analyse the rich kinematic substructure of the disc in the emph{Gaia} DR2
radial velocity sample (RVS). We revisit the strong perturbation induced in the
Milky Way (MW) disc by an $m=2$ bar, using test particle simulations and the
actions $(J_R,L_z,J_z)$ estimated in an axisymmetric potential. These make
three useful diagnostics cleanly visible. (1.) We use the well-known
characteristic flip from outward to inward motion at the Outer Lindblad
Resonance (OLR, $l=+1,m=2$), which occurs along the axisymmetric resonance line
(ARL) in $(L_z,J_R)$, to identify in the emph{Gaia} action data three
candidates for the bar’s OLR and pattern speed $Omega_text{bar}$:
$1.85Omega_0$, $1.20Omega_0$, and $1.63Omega_0$ (with $sim0.1Omega_0$
systematic uncertainty). The emph{Gaia} data is therefore consistent with both
slow and fast bar models in the literature, but disagrees with recent
measurements of $sim1.45Omega_0$. (2.) For the first time, we demonstrate
that bar resonances — especially the OLR — cause a gradient in vertical
action $langle J_z rangle$ with $L_z$ around the ARL via “$J_z$-sorting” of
stars. This could contribute to the observed coupling of $langle v_R rangle$
and $langle | v_z | rangle$ in the Galactic disc. (3.) We confirm prior
results that the behaviour of resonant orbits is well approximated by
scattering and oscillation in $(L_z,J_R)$ along a slope $Delta J_R/Delta L_z
= l/m$ centered on the $l$:$m$ ARL. Overall, we demonstrate that
axisymmetrically estimated actions are a powerful diagnostic tool even in
non-axisymmetric systems.

Action space synthesizes the orbital information of stars and is well-suited
to analyse the rich kinematic substructure of the disc in the emph{Gaia} DR2
radial velocity sample (RVS). We revisit the strong perturbation induced in the
Milky Way (MW) disc by an $m=2$ bar, using test particle simulations and the
actions $(J_R,L_z,J_z)$ estimated in an axisymmetric potential. These make
three useful diagnostics cleanly visible. (1.) We use the well-known
characteristic flip from outward to inward motion at the Outer Lindblad
Resonance (OLR, $l=+1,m=2$), which occurs along the axisymmetric resonance line
(ARL) in $(L_z,J_R)$, to identify in the emph{Gaia} action data three
candidates for the bar’s OLR and pattern speed $Omega_text{bar}$:
$1.85Omega_0$, $1.20Omega_0$, and $1.63Omega_0$ (with $sim0.1Omega_0$
systematic uncertainty). The emph{Gaia} data is therefore consistent with both
slow and fast bar models in the literature, but disagrees with recent
measurements of $sim1.45Omega_0$. (2.) For the first time, we demonstrate
that bar resonances — especially the OLR — cause a gradient in vertical
action $langle J_z rangle$ with $L_z$ around the ARL via “$J_z$-sorting” of
stars. This could contribute to the observed coupling of $langle v_R rangle$
and $langle | v_z | rangle$ in the Galactic disc. (3.) We confirm prior
results that the behaviour of resonant orbits is well approximated by
scattering and oscillation in $(L_z,J_R)$ along a slope $Delta J_R/Delta L_z
= l/m$ centered on the $l$:$m$ ARL. Overall, we demonstrate that
axisymmetrically estimated actions are a powerful diagnostic tool even in
non-axisymmetric systems.

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