The Stellar Halo of the Galaxy is Tilted & Doubly Broken

Abstract

Modern Galactic surveys have revealed an ancient merger that dominates the stellar halo of our Galaxy (\textit{Gaia}-Sausage-Enceladus, GSE). Using chemical abundances and kinematics from the H3 Survey, we identify 5559 halo stars from this merger in the radial range $r_{\text{Gal}}=6-60\text{ kpc}$. We forward model the full selection function of H3 to infer the density profile of this accreted component of the stellar halo. We consider a general ellipsoid with principal axes allowed to rotate with respect to the Galactocentric axes, coupled with a multiply-broken power law. The best-fit model is a triaxial ellipsoid (axes ratios 10:8:7) tilted $25^{\circ }$ above the Galactic plane towards the Sun and a doubly-broken power law with breaking radii at 12 kpc and 28 kpc. This result resolves the long-standing dichotomy in literature values of the halo breaking radius, being at either $\sim 15\text{ kpc}$ or $\sim 30\text{ kpc}$ assuming a singly-broken power law. N-body simulations suggest that the breaking radii are connected to apocenter pile-ups of stellar orbits, and so the observed double-break provides new insight into the initial conditions and evolution of the GSE merger. Furthermore, the tilt and triaxiality of the stellar halo could imply that a fraction of the underlying dark matter halo is also tilted and triaxial. This has important implications for dynamical mass modeling of the Galaxy as well as direct dark matter detection experiments.

arXiv commentSubmitted to ApJ; comments warmly welcome

References in corpus (84)

• emcee: The MCMC Hammer
• On the variation of the Initial Mass Function
• The Gaia mission
• MESA Isochrones and Stellar Tracks (MIST). I: Solar-Scaled Models
• MESA Isochrones and Stellar Tracks (MIST) 0: Methods for the construction of stellar isochrones
• The Apache Point Observatory Galactic Evolution Experiment (APOGEE)
• First results from the IllustrisTNG simulations: the stellar mass content of groups and clusters of galaxies
• The Galaxy in Context: Structural, Kinematic and Integrated Properties
• galpy: A Python Library for Galactic Dynamics
• The Milky Way Tomography with SDSS: I. Stellar Number Density Distribution
• The merger that led to the formation of the Milky Way's inner stellar halo and thick disk
• The New Galaxy: Signatures of its Formation
• A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: I. Morphology of the Sagittarius Core and Tidal Arms
• Tracing Galaxy Formation with Stellar Halos I: Methods
• Co-formation of the Galactic disc and the stellar halo
• The Field of Streams: Sagittarius and its Siblings
• The Ghost of Sagittarius and Lumps in the Halo of the Milky Way
• The Fourteenth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the extended Baryon Oscillation Spectroscopic Survey and from the second phase of the Apache Point Observatory Galactic Evolution Experiment
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• The spatial structure of mono-abundance sub-populations of the Milky Way disk
• The accretion origin of the Milky Way's stellar halo
• Streams, substructures and the early history of the Milky Way
• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• 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
• Detection of a 63 Degree Cold Stellar Stream in the Sloan Digital Sky Survey
• The Milky Way stellar halo out to 40 kpc : Squashed, broken but smooth
• The Biggest Splash
• The origin of accreted stellar halo populations in the Milky Way using APOGEE, $\text{Gaia}$, and the EAGLE simulations
• Galactic Halo substructure in the Sloan Digital Sky Survey: the ancient tidal stream from the Sagittarius dwarf galaxy
• Substructure revealed by RR Lyraes in SDSS Stripe 82
• The birth of the Milky Way as uncovered by accurate stellar ages with Gaia
• The Milky Way's Stellar Disk
• The QUEST RR Lyrae Survey: Confirmation of the Clump at 50 kpc and Other Over-Densities in the Outer Halo
• Light Curve Templates and Galactic Distribution of RR Lyrae Stars from Sloan Digital Sky Survey Stripe 82
• From dawn till disk: Milky Way's turbulent youth revealed by the APOGEE+Gaia data
• Gaia reveals a metal-rich in-situ component of the local stellar halo
• Hunting for the Dark Matter Wake Induced by the Large Magellanic Cloud
• The Hercules-Aquila Cloud
• The Shape and Profile of the Milky Way Halo as Seen by the CFHT Legacy Survey
• Machine-Learned Identification of RR Lyrae Stars from Sparse, Multi-band Data: the PS1 Sample
• Reconstructing the Last Major Merger of the Milky Way with the H3 Survey
• Halo Mass and Assembly History Exposed in the Faint Outskirts: the Stellar and Dark Matter Haloes of Illustris Galaxies
• Mapping the Stellar Halo with the H3 Spectroscopic Survey
• Ages and kinematics of chemically selected, accreted Milky Way halo stars
• The total stellar halo mass of the Milky Way
• The cold veil of the Milky Way stellar halo
• The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
• Apocenter Pile-Up: Origin of the Stellar Halo Density Break
• The shape of the Galactic halo with $Gaia$ DR2 RR Lyrae. Anatomy of an ancient major merger
• Origin of chemically distinct discs in the Auriga cosmological simulations
• Chronologically dating the early assembly of the Milky Way
• The radial profile and flattening of the Milky Way's stellar halo to $80$kpc from the SEGUE K-giant Survey
• Resolving the Metallicity Distribution of the Stellar Halo with the H3 Survey
• All-Sky Dynamical Response of the Galactic Halo to the Large Magellanic Cloud
• The SkyMapper-Gaia RVS view of the Gaia-Enceladus-Sausage -- an investigation of the metallicity and mass of the Milky Way's last major merger
• Timing the Early Assembly of the Milky Way with the H3 Survey
• Exploring the Variable Sky with LINEAR. II. Halo Structure and Substructure Traced by RR Lyrae Stars to 30 kpc
• 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
• Inferred Evidence For Dark Matter Kinematic Substructure with SDSS-Gaia
• Origin of the excess of high-energy retrograde stars in the Galactic halo
• The origin of diverse $\alpha$-element abundances in galaxy discs
• Galactic Archaeology. The dwarfs that survived and perished
• Touching The Void: A Striking Drop in Stellar Halo Density Beyond 50 kpc
• The first all-sky view of the Milky Way stellar halo with Gaia+2MASS RR Lyrae
• Age dating of an early Milky Way merger via asteroseismology of the naked-eye star $\nu$ Indi
• What's Up in the Milky Way? The Orientation of the Disc Relative to the Triaxial Halo
• Equilibrium models of the Milky Way mass are biased high by the LMC
• Common origin for Hercules-Aquila and Virgo Clouds in Gaia DR2
• MINESweeper: Spectrophotometric Modeling of Stars in the Gaia Era
• Characterising stellar halo populations I: An extended distribution function for halo K giants
• Merger-induced galaxy transformations in the ARTEMIS simulations
• The Profile of the Galactic Halo from Pan-STARRS1 3$\pi$ RR Lyrae
• Dark matter halo shapes in the Auriga simulations
• The stability of stellar disks in Milky-Way sized dark matter halos
• Creating mock catalogues of stellar haloes from cosmological simulations
• A skewer survey of the Galactic halo from deep CFHT and INT images
• Age-Dating Red Giant Stars Associated with Galactic Disk and Halo Substructures
• RR Lyrae in XSTPS: The halo density profile in the North Galactic Cap
• The twisted dark matter halo of the Milky Way
• Age Determination of Galaxy Merger Remnant Stars using Asteroseismology
• Inferring the Morphology of Stellar Distribution in TNG50: Twisted and Twisted-Stretched shapes
• They Might Be Giants: An Efficient Color-Based Selection of Red Giant Stars
• Influence of the Gaia-Sausage-Enceladus on the density shape of the Galactic stellar halo revealed by halo K giants from the LAMOST survey

Cited by in corpus (24)

• Galactic Archaeology with Gaia
• Energy wrinkles and phase-space folds of the last major merger
• A Tilted Dark Halo Origin of the Galactic Disk Warp and Flare
• Distant Echoes of the Milky Way's Last Major Merger
• Mapping the anisotropic Galactic stellar halo with Blue Horizontal Branch stars
• Constraining the Gravitational Potential from the Projected Morphology of Extragalactic Tidal Streams
• VINTERGATAN-GM: The cosmological imprints of early mergers on Milky-Way-mass galaxies
• A Swing of the Pendulum: The Chemodynamics of the Local Stellar Halo Indicate Contributions from Several Radial Merger Events
• Milky Way-est: Cosmological Zoom-in Simulations with Large Magellanic Cloud and Gaia-Sausage-Enceladus Analogs
• Exploring the diversity and similarity of radially anisotropic Milky Way-like stellar haloes: implications for disrupted dwarf galaxy searches
• All-Sky Kinematics of the Distant Halo: The Reflex Response to the LMC
• Ironing the folds: The phase space chevrons of a GSE-like merger as a dark matter subhalo detector
• Mapping the Distant and Metal-Poor Milky Way with SDSS-V
• From order to chaos: the blurred out metallicity gradient of the Gaia-Enceladus/Sausage progenitor
• Revealing the Milky Way's Most Recent Major Merger with a Gaia EDR3 Catalog of Machine-Learned Line-of-Sight Velocities
• Shapes of dark matter haloes with discrete globular cluster dynamics: The example of NGC 5128 (Centaurus A)
• A Vertically Orientated Dark Matter Halo Marks a Flip of the Galactic Disk
• Discovery of the shell structure via break radii in the outer halo of the Milky Way
• Stellar halo striations from assumptions of axisymmetry
• Build-up and survival of the disc: From numerical models of galaxy formation to the Milky Way
• Quantifying the Milky Way, LMC and their interaction using all-sky kinematics of outer halo stars
• GSE vs. LMC: reshaping of radially biased stellar haloes by satellites
• Many body gravity and the galaxy rotation curves
• photoD with Rubin's Data Preview 1: first stellar photometric distances and deficit of faint blue stars. Stellar distances with Rubin's DP1