The tilt of the velocity ellipsoid in the Milky Way with Gaia DR2

Abstract

The velocity distribution of stars is a sensitive probe of the gravitational potential of the Galaxy, and hence of its dark matter distribution. In particular, the shape of the dark halo (e.g. spherical, oblate, or prolate) determines velocity correlations, and different halo geometries are expected to result in measurable differences. Here we explore and interpret the correlations in the $(v_R,v_z)$-velocity distribution as a function of position in the Milky Way. We selected a high-quality sample of stars from the Gaia DR2 catalogue and characterised the orientation of the velocity distribution or tilt angle over a radial distance range of $[4-13]$kpc and up to $3.5$kpc away from the Galactic plane while taking into account the effects of the measurement errors. We find that the tilt angles change from spherical alignment in the inner Galaxy ($R\sim 4$kpc) towards more cylindrical alignments in the outer Galaxy ($R\sim 11$kpc) when using distances that take a global zero-point offset in the parallax of $-29\mu$as. However, if the amplitude of this offset is underestimated, then the inferred tilt angles in the outer Galaxy only appear shallower and are intrinsically more consistent with spherical alignment for an offset as large as $-54\mu$as. We further find that the tilt angles do not seem to strongly vary with Galactic azimuth and that different stellar populations depict similar tilt angles. Therefore we introduce a simple analytic function that describes the trends found over the full radial range. Since the systematic parallax errors in Gaia DR2 depend on celestial position, magnitude, and colour in complex ways, it is not possible to fully correct for them. Therefore it will be particularly important for dynamical modelling of the Milky Way to thoroughly characterise the systematics in astrometry in future Gaia data releases.

arXiv comment15 pages, 13 figures, A&A

References in corpus (59)

• emcee: The MCMC Hammer
• The Sloan Digital Sky Survey: Technical Summary
• Gaia Data Release 2. Summary of the contents and survey properties
• The Structure of Cold Dark Matter Halos
• The Seventh Data Release of the Sloan Digital Sky Survey
• Gaia Data Release 1. Summary of the astrometric, photometric, and survey properties
• Gaia Data Release 2: The astrometric solution
• Local Kinematics and the Local Standard of Rest
• The Apache Point Observatory Galactic Evolution Experiment (APOGEE)
• Trigonometric Parallaxes of High Mass Star Forming Regions: the Structure and Kinematics of the Milky Way
• The merger that led to the formation of the Milky Way's inner stellar halo and thick disk
• SEGUE: A Spectroscopic Survey of 240,000 stars with g=14-20
• The mass distribution and gravitational potential of the Milky Way
• Co-formation of the Galactic disc and the stellar halo
• A geometric distance measurement to the Galactic Center black hole with 0.3% uncertainty
• Mass models of the Milky Way
• Gaia Data Release 2: Kinematics of globular clusters and dwarf galaxies around the Milky Way
• Gaia Data Release 2: Catalogue validation
• The Radial Velocity Experiment (RAVE): Fifth Data Release
• A dynamically young and perturbed Milky Way disk
• Gaia Data Release 2: Mapping the Milky Way disc kinematics
• The Distribution of Nearby Stars in Velocity Space Inferred from Hipparcos Data
• The photometric structure of the inner Galaxy
• Galactic Rotation and Solar Motion from Stellar Kinematics
• The wobbly Galaxy: kinematics north and south with RAVE red clump giants
• The Milky Way Tomography with SDSS: III. Stellar Kinematics
• Gaia Universe Model Snapshot : A statistical analysis of the expected contents of the Gaia catalogue
• The Spur and the Gap in GD-1: Dynamical evidence for a dark substructure in the Milky Way halo
• 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
• Distances and parallax bias in Gaia DR2
• Models of our Galaxy II
• Characterization and history of the Helmi streams with Gaia DR2
• Origin and evolution of moving groups I. Characterization in the observational kinematic-age-metallicity space
• Off the beaten path: Gaia reveals GD-1 stars outside of the main stream
• The Galactic warp revealed by Gaia DR2 kinematics
• The Gravitational Force Field of the Galaxy Measured From the Kinematics of RR Lyrae in Gaia
• Estimation of the Tilt of the Stellar Velocity Ellipsoid from RAVE and Implications for Mass Models
• Slicing and dicing the Milky Way disk in SDSS
• Galactic kinematics and dynamics from RAVE stars
• High Resolution Chandra HETG spectroscopy of V404 Cygni in Outburst
• Vaex: Big Data exploration in the era of Gaia
• The Tilt of the Halo Velocity Ellipsoid and the Shape of the Milky Way Halo
• The 3D Velocity Structure of the Thick Disk from SPM4 and RAVE DR2
• Is the Milky Way still breathing? RAVE-Gaia streaming motions
• New light on the Gaia DR2 parallax zero-point: influence of the asteroseismic approach, in and beyond the Kepler field
• Testing asteroseismology with Gaia DR2: Hierarchical models of the Red Clump
• Assessing distances and consistency of kinematics in Gaia/TGAS
• The tilt of the velocity ellipsoid in the Milky Way disk
• The dynamically selected stellar halo of the Galaxy with Gaia and the tilt of the velocity ellipsoid
• The detection and treatment of distance errors in kinematic analyses of stars
• Three-component Stackel potentials satisfying recent estimates of Milky Way parameters
• Asteroseismic radii of dwarfs: New accuracy constraints from Gaia DR2 parallaxes
• General solution of the Jeans equations for triaxial galaxies with separable potentials
• The Tilt of the Local Velocity Ellipsoid as Seen by Gaia
• The stellar kinematics in the solar neighborhood from LAMOST data
• The peculiar velocities in the Galactic outer disk--hints of the elliptical disk and the perturbation of the spiral structures
• The peculiar in-plane velocities in the outer disc of the Milky Way
• What can the alignments of the velocity moments tell us about the nature of the potential?

Cited by in corpus (14)

• From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity
• Efficient solution of the anisotropic spherically-aligned axisymmetric Jeans equations of stellar hydrodynamics for galactic dynamics
• Measuring the local dark matter density with LAMOST DR5 and Gaia DR2
• The SAMI Galaxy Survey: The internal orbital structure and mass distribution of passive galaxies from triaxial orbit-superposition Schwarzschild models
• First Gaia dynamical model of the Milky Way disc with six phase space coordinates: a test for galaxy dynamics
• Dynamical model of the Milky Way using APOGEE and Gaia data
• On the Galactic rotation curve inferred from the Jeans equations Assessing its robustness using Gaia DR3 and cosmological simulations
• Vertical structure of Galactic disk kinematics from LAMOST K giants
• The tilt of the velocity ellipsoid of different Galactic disk populations
• General model of vertical distribution of stars in the Milky Way using complete Jeans equations
• The global structure of the Milky Way's stellar halo based on the orbits of local metal-poor stars
• Local variations of the Stellar Velocity Ellipsoid-I: the disc of galaxies in the Auriga simulations
• The 3D Galactocentric velocities of Kepler stars: marginalizing over missing RVs
• Asymmetries in random motions of neutral Hydrogen gas in spiral galaxies