Action space synthesizes the orbital information of stars and is well-suited to analyse the rich amount of kinematic disk substructure in the Gaia DR2 radial velocity sample (RVS). In this work, we revisit one of the strongest perturbers in the Milky Way (MW) disk: the m=2 bar. We investigate how its resonances affect the actions of individual test particle stars, i.e., (JR,Lz,Jz) estimated in an axisymmetric MW potential. We confirm that the stars' behaviour is well approximated by scattering and oscillation along a slope Delta JR / Delta Lz = l/m centered on the l:m resonance lines. The Outer Lindblad Resonance (OLR, l=+1,m=2) creates signatures in the stellar action space that can be used to identify the Galactic bar's OLR in the Gaia DR2 RVS data: (a) The JR dependence of the oscillation causes an overdensity ridge (underdensity region) at Lz larger (smaller) than the resonance line in the (Lz,JR) plane. (b) For the first time, we demonstrate that the OLR is expected to cause a gradient in average Jz with Lz across the resonance. (c) We show that the change of predominantly outward to inward motions at the OLR occurs along the resonance line in action space. The latter signature allows us to identify three candidates for the bar's OLR - and therefore its pattern speed Omega_bar - in the Gaia data within 3 kpc from the Sun: 1.85 Omega0, 1.2 Omega0, and 1.6 Omega0 (with ~0.1 Omega0 uncertainty). This demonstrates that (i) the local Gaia action data is consistent with both the short-fast and long-slow bar models in the literature, and that (ii) axisymmetrically estimated actions are a powerful diagnostic even in non-axisymmetric systems.

中文翻译：

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识别 Gaia DR2 中银河棒的共振：来自行动空间的线索

行动空间综合了恒星的轨道信息，非常适合分析 Gaia DR2 径向速度样本 (RVS) 中丰富的运动盘子结构。在这项工作中，我们重新审视了银河系 (MW) 圆盘中最强的扰动体之一：m=2 bar。我们研究了它的共振如何影响单个测试粒子星的行为，即在轴对称 MW 势中估计的 (JR,Lz,Jz)。我们确认恒星的行为可以通过沿着以 l:m 共振线为中心的斜率 Delta JR / Delta Lz = l/m 的散射和振荡来很好地近似。外部 Lindblad 共振 (OLR, l=+1,m=2) 在恒星作用空间中创建特征，可用于识别 Gaia DR2 RVS 数据中银河棒的 OLR：(a) 振荡的 JR 依赖性导致 Lz 处的高密度脊（低密度区域）大于（小于）（Lz，JR）平面中的共振线。(b) 我们第一次证明 OLR 预计会导致平均 Jz 的梯度，其中 Lz 跨越共振。(c) 我们表明，OLR 处的主要向外向向内运动的变化沿动作空间中的共振线发生。后一个签名使我们能够在距太阳 3 kpc 范围内的盖亚数据中确定柱的 OLR 的三个候选者 - 因此其模式速度 Omega_bar：1.85 Omega0、1.2 Omega0 和 1.6 Omega0（具有 ~0.1 Omega0 不确定性）。这表明 (i) 本地 Gaia 动作数据与文献中的短快和长慢条模型一致，