The shape of the Galactic halo with $Gaia$ DR2 RR Lyrae. Anatomy of an ancient major merger

Abstract

We use the $Gaia$ DR2 RR Lyrae sample to gain an uninterrupted view of the Galactic stellar halo. We dissect the available volume in slices parallel to the Milky Way's disc to show that within $\sim 30$ kpc from the Galactic centre the halo is triaxial, with the longest axis misaligned by $\sim 70^{\circ }$ with respect to the Galactic $x$-axis. This anatomical procedure exposes two large diffuse over-densities aligned with the semi-major axis of the halo: the Hercules-Aquila Cloud and the Virgo Over-density. We reveal the kinematics of the entire inner halo by mapping out the amplitudes and directions of the RR Lyrae proper motions. These are then compared to simple models with different anisotropies to demonstrate that the inner halo is dominated by stars on highly eccentric orbits. We interpret the shape of the density and the kinematics of the $Gaia$ DR2 RR Lyrae as evidence in favour of a scenario in which the bulk of the halo was deposited in a single massive merger event.

12 pages, 7 figures; Accepted for publication in MNRAS

References in corpus (44)

• Gaia Data Release 2. Summary of the contents and survey properties
• The Gaia mission
• Gaia Data Release 2: The astrometric solution
• Estimating distances from parallaxes IV: Distances to 1.33 billion stars in Gaia Data Release 2
• The merger that led to the formation of the Milky Way's inner stellar halo and thick disk
• The mass distribution and gravitational potential of the Milky Way
• Co-formation of the Galactic disc and the stellar halo
• Gaia Data Release 2: Photometric content and validation
• Gaia Data Release 2: Kinematics of globular clusters and dwarf galaxies around the Milky Way
• Two Stellar Components in the Halo of the Milky Way
• Properties of Dark Matter Haloes in Clusters, Filaments, Sheets and Voids
• The spin and shape of dark matter haloes in the Millennium simulation of a LambdaCDM universe
• The accretion origin of the Milky Way's stellar halo
• The formation and assembly history of the Milky Way revealed by its globular cluster population
• In disguise or out of reach: first clues about in-situ and accreted stars in the stellar halo of the Milky Way from Gaia DR2
• The Dual Origin of Stellar Halos
• On the local dark matter density
• The origin of accreted stellar halo populations in the Milky Way using APOGEE, $\text{Gaia}$, and the EAGLE simulations
• "Skinny Milky Way, Please", says Sagittarius
• The Galaxy and its stellar halo: insights on their formation from a hybrid cosmological approach
• The Sausage Globular Clusters
• The Causes of Halo Shape Changes Induced by Cooling Baryons: Disks Versus Substructures
• The Auriga Stellar Haloes: Connecting stellar population properties with accretion and merging history
• Kinematics of the stellar halo and the mass distribution of the Milky Way using BHB stars
• The shape of the inner Milky Way halo from observations of the Pal 5 and GD-1 stellar streams
• The Hercules-Aquila Cloud
• The Shape of the Gravitational Potential in Cold Dark Matter Halos
• Disentangling the Galactic Halo with APOGEE: I. Chemical and Kinematical Investigation of Distinct Metal-Poor Populations
• RR Lyrae stars as standard candles in the Gaia Data Release 2 Era
• Apocenter Pile-Up: Origin of the Stellar Halo Density Break
• Clouds, Streams and Bridges. Redrawing the blueprint of the Magellanic System with Gaia DR1
• The Milky Way Halo in Action Space
• Inferred Evidence For Dark Matter Kinematic Substructure with SDSS-Gaia
• The Gravitational Force Field of the Galaxy Measured From the Kinematics of RR Lyrae in Gaia
• A New Catalog of Globular Clusters in the Milky Way
• The Halo's Ancient Metal-Rich Progenitor Revealed with BHB Stars
• The Influence of Large-Scale Structure on Halo Shapes and Alignments
• Dipping Our Toes in the Water: First Models of GD-1 as a Stream
• The first all-sky view of the Milky Way stellar halo with Gaia+2MASS RR Lyrae
• Gaia Data Release 2: Summary of the variability processing & analysis results
• Common origin for Hercules-Aquila and Virgo Clouds in Gaia DR2
• Anisotropy of the Milky Way's stellar halo using K giants from LAMOST and $Gaia$
• First Gaia Local Group Dynamics: Magellanic Clouds Proper Motion and Rotation
• Unmixing the Galactic Halo with RR Lyrae tagging

Cited by in corpus (100)

• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• Streams, substructures and the early history of the Milky Way
• Unresolved stellar companions with Gaia DR2 astrometry
• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• The Zwicky Transient Facility Catalog of Periodic Variable Stars
• The Biggest Splash
• Tango for three: Sagittarius, LMC, and the Milky Way
• What is a globular cluster? An observational perspective
• The Milky Way has no in-situ halo other than the heated thick disc. Composition of the stellar halo and age-dating the last significant merger with Gaia DR2 and APOGEE
• From dawn till disk: Milky Way's turbulent youth revealed by the APOGEE+Gaia data
• The hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2
• Dark matter local density determination: recent observations and future prospects
• Reconstructing the Last Major Merger of the Milky Way with the H3 Survey
• Overview of the DESI Milky Way Survey
• Dynamical Relics of the Ancient Galactic Halo
• The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
• The origin of galactic metal-rich stellar halo components with highly eccentric orbits
• Piercing the Milky Way: an all-sky view of the Orphan Stream
• The Milky Way bar and bulge revealed by APOGEE DR16 and Gaia EDR3
• Resolving the Metallicity Distribution of the Stellar Halo with the H3 Survey
• Detection of the Milky Way reflex motion due to the Large Magellanic Cloud infall
• Weighing the stellar constituents of the Galactic halo with APOGEE red giant stars
• Chemo-kinematics of the $Gaia$ RR Lyrae: the halo and the disc
• The Gravitational Force Field of the Galaxy Measured From the Kinematics of RR Lyrae in Gaia
• Reconstructing the Disrupted Dwarf Galaxy $Gaia$-Sausage/Enceladus Using its Stars and Globular Clusters
• Evidence of a dynamically evolving Galactic warp
• Stellar migrations and metal flows -- Chemical evolution of the thin disc of a simulated Milky Way analogous galaxy
• Galactic Archaeology with Gaia
• Directional recoil detection
• The Stellar Halo of the Galaxy is Tilted & Doubly Broken
• The Pisces Plume and the Magellanic Wake
• Merger-induced galaxy transformations in the ARTEMIS simulations
• The tidal remnant of an unusually metal-poor globular cluster
• A panoramic landscape of the Sagittarius stream in Gaia DR2 revealed with the STREAMFINDER spyglass
• Photometric metallicity and distance estimates for $\sim$ 136,000 RR Lyrae stars from Gaia DR3
• Constraints on the assembly history of the Milky Way's smooth, diffuse stellar halo from the metallicity-dependent, radially-dominated velocity anisotropy profiles probed with K giants and BHB stars using LAMOST, SDSS/SEGUE, and Gaia
• The contribution of N-rich stars to the Galactic stellar halo using APOGEE red giants
• Energy wrinkles and phase-space folds of the last major merger
• Predicting accreted satellite galaxy masses and accretion redshifts based on globular cluster orbits in the E-MOSAICS simulations
• The Milky Way's Shell Structure Reveals the Time of a Radial Collision
• First results on RR Lyrae stars with the TESS space telescope: untangling the connections between mode content, colors and distances
• Kinematics of the Palomar 5 stellar stream from RR Lyrae stars
• Dark Matter Interpretation of the Fermi-LAT Observations Toward the Outer Halo of M31
• RR Lyrae From Binary Evolution: Abundant, Young and Metal-Rich
• Action-based distribution function modelling for constraining the shape of the Galactic dark matter halo
• Magellanic Mayhem: Metallicities and Motions
• The Local Stellar Halo is Not Dominated by a Single Radial Merger Event
• Local RR Lyrae Stars: native and alien
• The kinematics of globular cluster populations in the E-MOSAICS simulations and their implications for the assembly history of the Milky Way
• Distant Echoes of the Milky Way's Last Major Merger
• The Virgo Overdensity Explained
• The Magellanic Edges Survey I. Description and First Results
• Cosmological insights into the assembly of the radial and compact stellar halo of the Milky Way
• The impact of a massive Sagittarius dSph on GD-1-like streams
• The Unmixed Debris of Gaia-Sausage/Enceladus in the Form of a Pair of Halo Stellar Overdensities
• The observable properties of galaxy accretion events in Milky Way-like galaxies in the FIRE-2 cosmological simulations
• The Cetus-Palca stream: A disrupted small dwarf galaxy. A prequel to the science possible with WEAVE with precise spectro-photometric distances
• Variable star classification across the Galactic bulge and disc with the VISTA Variables in the Vía Láctea survey
• Probing the Galactic halo with RR Lyrae stars $-$ III. The chemical and kinematic properties of the stellar halo
• Exploring the diversity and similarity of radially anisotropic Milky Way-like stellar haloes: implications for disrupted dwarf galaxy searches
• Gamma-ray emission from the Sagittarius Dwarf Spheroidal galaxy due to millisecond pulsars
• Completeness of the Gaia-verse V: Astrometry and Radial Velocity sample selection functions in Gaia EDR3
• Three-dimensional structure of the Sagittarius dSph core from RR Lyrae
• Tidally stripped halo stars from the Large Magellanic Cloud in the Galactic North
• A panoramic view of the Local Group dwarf galaxy NGC 6822
• Detecting Globular Cluster Tidal Extensions with Bayesian Inference: I. Analysis of $\omega$ Centauri with Gaia EDR3
• Linking nearby stellar streams to more distant halo overdensities
• Inferring the Morphology of Stellar Distribution in TNG50: Twisted and Twisted-Stretched shapes
• A 6-d View of Stellar Shells
• Explaining the chemical trajectories of accreted and in-situ halo stars of the Milky Way
• On the Mass Assembly History of the Local Group
• The Photo-Astrometric Vertical Tracer Density of the Milky Way II: Results from Gaia
• Measuring Galactic Dark Matter through Unsupervised Machine Learning
• Ironing the folds: The phase space chevrons of a GSE-like merger as a dark matter subhalo detector
• The Milky Way Revealed by Variable Stars I: Sample Selection of RR Lyrae stars, and Evidence for the Merger History
• The Early Merger that Made the Galaxy's Stellar Halo
• Influence of the Gaia-Sausage-Enceladus on the density shape of the Galactic stellar halo revealed by halo K giants from the LAMOST survey
• The Reduced Proper Motion selected halo: methods and description of the catalogue
• Applying Noether's theorem to matter in the Milky Way: evidence for external perturbations and non-steady-state effects from Gaia Data Release 2
• Non-Parametric Spherical Jeans Mass Estimation with B-splines
• Calibrating the BHB Star Distance Scale and the Halo Kinematic Distance to the Galactic Centre
• Formation of Transitional cE/UCD Galaxies through Massive/Dwarf Disc Galaxy Mergers
• Accelerated phase-mixing in the stellar halo due to a rotating bar
• The global structure of the Milky Way's stellar halo based on the orbits of local metal-poor stars
• Photometric-Metallicity and Distance Estimates for $\sim$70,000 RR Lyrae Stars from the Zwicky Transient Facility
• Revealing the Milky Way's Most Recent Major Merger with a Gaia EDR3 Catalog of Machine-Learned Line-of-Sight Velocities
• Determining the Scale Length and Height of the Milky Way's Thick Disc Using RR~Lyrae
• The Pristine Survey -- XXVII. Journey to the Galactic outskirts -- Mapping the outer halo red giant stars down to the very metal-poor end
• Discovery of the shell structure via break radii in the outer halo of the Milky Way
• Investigating stellar variability in the open cluster region NGC 381
• Shapes of dark matter haloes with discrete globular cluster dynamics: The example of NGC 5128 (Centaurus A)
• Tomography of stellar halos: what does anisotropy in a stellar halo tell us?
• On the Use of Field RR Lyrae as Galactic Probes -- VIII. Early Formation of the Galactic Spheroid
• Stellar halo striations from assumptions of axisymmetry
• Numerical effects on the stripping of dark matter and stars in IllustrisTNG galaxy groups and clusters
• Features of the Residual Velocity Ellipsoid of Hot Subdwarfs from the Gaia DR2 Catalog
• GSE vs. LMC: reshaping of radially biased stellar haloes by satellites
• Probing the spatial and velocity anisotropies in stellar halos from the Aquarius simulations
• Tracing Milky Way substructure with an RR Lyrae hierarchical clustering forest
• A Tilt in the Dark Matter Halo of the Galaxy