Identifying resonances of the Galactic bar in Gaia DR2: II. Clues from angle space

Abstract

The Milky Way disk exhibits intricate orbit substructure of still-debated dynamical origin. The angle variables $({\theta }_{\varphi },{\theta }_R)$ -- which are conjugates to the actions $(L_z,J_R)$, and describe a star's location along its orbit -- are a powerful diagnostic to identify $l$:$m$ resonances via the orbit shape relation $\Delta {\theta }_R/\Delta {\theta }_{\varphi }=-m/l$. In the past, angle signatures have been hidden by survey selection effects (SEs). Using test particle simulations of a barred galaxy, we demonstrate that \emph{Gaia} should allow us to identify the Galactic bar's Outer Lindblad Resonance ($l=+1,m=2$, OLR) in angle space. We investigate strategies to overcome SEs. In the angle data of the \emph{Gaia} DR2 RVS sample, we independently identify four candidates for the OLR and therefore for the pattern speed ${\Omega }_{\text{bar}}$. The strongest candidate, ${\Omega }_{\text{bar}}\sim 1.4{\Omega }_0$, positions the OLR above the `Sirius' moving group, agrees with measurements from the Galactic center, and might be supported by higher-order resonances around the `Hercules/Horn'. But it misses the classic orbit orientation flip, as discussed in the companion study on actions. The candidate ${\Omega }_{\text{bar}}\sim 1.2{\Omega }_0$ was also suggested by the action-based study, has the OLR at the `Hat', is consistent with \emph{slow bar} models, but still affected by SEs. Weaker candidates are ${\Omega }_{\text{bar}}=1.6{\Omega }_0$ and $1.74{\Omega }_0$. In addition, we show that the stellar angles do not support the `Hercules/Horn' being created by the OLR of a \emph{fast bar}. We conclude that -- to resolve if `Sirius' or `Hat' is related to the bar's OLR -- more complex dynamical explanations and more extended data with well-behaved SEs are required.

16 pages, 9 figures (excluding appendices); accepted for publication in MNRAS

References in corpus (29)

• The Gaia mission
• galpy: A Python Library for Galactic Dynamics
• Dynamics of Barred Galaxies
• Dynamical modelling of the Galactic bulge and bar: the Milky Way's bar pattern speed, stellar, and dark matter mass distribution
• The Radial Velocity Experiment (RAVE): second data release
• Distances and parallax bias in Gaia DR2
• The Hyades stream: evaporated cluster or intrusion from the inner disk?
• Stellar Kinematic Constraints on Galactic Structure Models Revisited: Bar and Spiral Arm Resonances
• The local spiral structure of the Milky Way
• Tree-ring structure of Galactic bar resonance
• On the ridges, undulations & streams in Gaia DR2: Linking the topography of phase-space to the orbital structure of an N-body bar
• Tracing the Hercules stream with Gaia and LAMOST: new evidence for a fast bar in the Milky Way
• Fourier Dissection of Early-Type Galaxy Bars
• Completeness of the Gaia-verse II: what are the odds that a star is missing from Gaia DR2?
• Phase Space Structure in the Solar Neighbourhood
• Hic sunt dracones: Cartography of the Milky Way spiral arms and bar resonances with Gaia Data Release 2
• Orbital structure in barred spiral galaxies
• Staying away from the bar: the local dynamical signature of slow and fast bars in the Milky Way
• Orbital tori for non-axisymmetric galaxies
• Trimodal structure of Hercules stream explained by originating from bar resonances
• The Effects of Boxy/Peanut Bulges on Galaxy Models
• Galactic Bar Resonances Inferred from Kinematically Hot Stars in Gaia EDR3
• Distribution functions for resonantly trapped orbits in the Galactic disc
• The peculiar in-plane velocities in the outer disc of the Milky Way
• Young stars as tracers of a barred-spiral Milky Way
• The power of co-ordinate transformations in dynamical interpretations of Galactic structure
• Characterising the Gaia Radial Velocity sample selection function in its native photometry
• Stars with fast Galactic rotation observed in Gaia TGAS: a signature driven by the Perseus arm?
• Axial Asymmetry Studies in Gaia Data Release 2 Yield the Pattern Speed of the Galactic Bar

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• Gaia Data Release 3: Chemical cartography of the Milky Way
• Stellar halo substructure generated by bar resonances
• Chemo-kinematics of the Milky Way spiral arms and bar resonances: connection to ridges and moving groups in the Solar vicinity
• Radial halo substructure in harmony with the Galactic bar
• Timing the Milky Way bar formation and the accompanying radial migration episode
• Spiral-like features in the disc revealed by Gaia DR3 radial actions
• Moving groups across Galactocentric radius with Gaia DR3
• Chemodynamical signatures of bar resonances in the Galactic disk: current data and future prospects
• A new resonance-like feature in the outer disc of the Milky Way
• Radial Wave in the Galactic Disk: New Clues to Discriminate Different Perturbations
• Impact of bar resonances in the velocity-space distribution of the solar neighbourhood stars in a self-consistent $N$-body Galactic disc simulation
• Radial and azimuthal gradients of the moving groups in Gaia DR3: The slow/fast bar degeneracy problem
• Trojan globular clusters: radial migration via trapping in bar resonances
• A non-axisymmetric potential for the Milky Way disk
• Dynamical streams in the local stellar halo
• Globular clusters in \textsc{OrbIT}: complete dynamical characterisation of the globular cluster population of the Milky Way through updated orbital reconstruction
• Moving Groups in the Solar Neighborhood with Gaia, APOGEE, GALAH, and LAMOST: Dynamical Effects Gather Gas and the Ensuing Star Formation Plays an Important Role in Shaping the Stellar Velocity Distributions
• Comparing radial migration in dark matter and MOND regimes
• GalPort: Investigation of the bar in action-angle space
• Bar Properties and Star-by-Star Bar Membership via Action Conservation