Probing Axial Symmetry Breaking in the Galaxy with Gaia Data Release 2

Abstract

We study a set of solar neighborhood ($d<3$ kpc) stars from Gaia Data Release 2 to determine azimuthal star count differences, i.e., left and right of the line from the Galactic center through the sun - and compare these differences north and south. In this companion paper to Gardner et al. (2020), we delineate our procedures to remove false asymmetries from sampling effects, incompleteness, and/or interloper populations, as this is crucial to tests of axisymmetry. Particularly, we have taken care to make appropriate selections of magnitude, color, in-plane Galactocentric radius and Galactic $|b|$ and $|z|$. We find that requiring parallax determinations of high precision induces sampling biases, so that we eschew such requirements and exclude, e.g., regions around the lines of sight to the Magellanic clouds, along with their mirror-image lines of sight, to ensure well-matched data sets. After making conservative cuts, we demonstrate the existence of azimuthal asymmetries, and find differences in those, north and south. These asymmetries give key insights into the nature and origins of the perturbations on Galactic matter, allowing us to assess the relative influence of the Magellanic Clouds (LMC & SMC), the Galactic bar, and other masses on the Galactic mass distribution, as described in Gardner et al. (2020). The asymmetry's radial dependence reveals variations that we attribute to the Galactic bar, and it changes sign at a radius of $(0.95\pm 0.03)R_0$, with $R_0$ the Sun-Galactic-Center (GC) distance, to give us the first direct assessment of the outer Lindblad resonant radius.

arXiv comment22 pages, 14 figures, accepted for publication in ApJ

References in corpus (29)

• A Universal Density Profile from Hierarchical Clustering
• The Gaia mission
• Gaia Data Release 2: The astrometric solution
• A geometric distance measurement to the Galactic Center black hole with 0.3% uncertainty
• Gaia Data Release 2: using Gaia parallaxes
• Gaia Data Release 2: Catalogue validation
• Estimating distances from parallaxes
• The Effect of the Outer Lindblad Resonance of the Galactic Bar on the Local Stellar Velocity Distribution
• A direct dynamical measurement of the Milky Way's disk surface density profile, disk scale length, and dark matter profile at 4 kpc < R < 9 kpc
• Vertical waves in the solar neighbourhood in Gaia DR2
• The total mass of the Large Magellanic Cloud from its perturbation on the Orphan stream
• Galactoseismology: Discovery of Vertical Waves in the Galactic Disk
• Actions for axisymmetric potentials
• The Sagittarius impact as an architect of spirality and outer rings in the Milky Way
• Stellar populations in the Milky Way bulge region : Towards solving the Galactic bulge and bar shapes using 2MASS data
• The astrometric core solution for the Gaia mission. Overview of models, algorithms and software implementation
• The Influence of Sagittarius and the Large Magellanic Cloud on the Milky Way Galaxy
• Constraining the Galaxy's dark halo with RAVE stars
• Confirmation of the $Gaia$ DR2 parallax zero-point offset using asteroseismology and spectroscopy in the $Kepler$ field
• Rotation curve of the Milky Way from Classical Cepheids
• A Magellanic Origin for the Warp of the Galaxy
• The Stellar Number Density Distribution in the Local Solar Neighborhood is North-South Asymmetric
• Galactic kinematics and dynamics from RAVE stars
• The distribution function of the Galaxy's dark halo
• Growth of a peanut shaped bulge via resonant trapping of stellar orbits in the vertical Inner Lindblad Resonances
• Bringing the Galaxy's dark halo to life
• Milky Way Tomography with K and M Dwarf Stars: the Vertical Structure of the Galactic Disk
• Gaia astrometry for stars with too few observations - a Bayesian approach
• Applying Noether's theorem to matter in the Milky Way: evidence for external perturbations and non-steady-state effects from Gaia Data Release 2

Cited by in corpus (6)

• Quantifying the impact of the Large Magellanic Cloud on the structure of the Milky Way's dark matter halo using Basis Function Expansions
• Identifying resonances of the Galactic bar in Gaia DR2: II. Clues from angle space
• The Milky Way, Coming into Focus: Precision Astrometry Probes its Evolution, and its Dark Matter
• Axial Asymmetry Studies in Gaia Data Release 2 Yield the Pattern Speed of the Galactic Bar
• Two-point correlation function studies for the Milky Way: discovery of spatial clustering from disk excitations and substructure
• A Wave-Corrected Assessment of the Local Midplane