Identifying resonances of the Galactic bar in Gaia DR2: I. Clues from action space

Wilma H. Trick; Francesca Fragkoudi; Jason A. S. Hunt; J. Ted Mackereth; Simon D. M. White

doi:10.1093/mnras/staa3317

paper

Identifying resonances of the Galactic bar in Gaia DR2: I. Clues from action space

arXiv:1906.04786 · doi:10.1093/mnras/staa3317

Abstract

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 $Ω_{bar}$ : $1.85 Ω_{0}$ , $1.20 Ω_{0}$ , and $1.63 Ω_{0}$ (with $\sim 0.1 Ω_{0}$ 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 $\sim 1.45 Ω_{0}$ . (2.) For the first time, we demonstrate that bar resonances -- especially the OLR -- cause a gradient in vertical action $⟨ J_{z} ⟩$ with $L_{z}$ around the ARL via " $J_{z}$ -sorting" of stars. This could contribute to the observed coupling of $⟨ v_{R} ⟩$ and $⟨ ∣ v_{z} ∣ ⟩$ 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 $Δ J_{R} /Δ L_{z} = l / m$ centered on the $l$ : $m$ ARL. Overall, we demonstrate that axisymmetrically estimated actions are a powerful diagnostic tool even in non-axisymmetric systems.

17 pages, 12 figures, 1 table (excluding appendices); accepted for publication in MNRAS; this replacement contains minor fixes to match published version

Identifying resonances of the Galactic bar in Gaia DR2: I. Clues from action space

Abstract

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