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.

Submitted to ApJ; comments warmly welcome

References in corpus (30)

• The Gaia mission
• galpy: A Python Library for Galactic Dynamics
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• The accretion origin of the Milky Way's stellar halo
• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• Detection of a 63 Degree Cold Stellar Stream in the Sloan Digital Sky Survey
• From dawn till disk: Milky Way's turbulent youth revealed by the APOGEE+Gaia data
• The Hercules-Aquila Cloud
• 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
• The total stellar halo mass of the Milky Way
• The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
• Origin of chemically distinct discs in the Auriga cosmological simulations
• All-Sky Dynamical Response of the Galactic Halo to the Large Magellanic Cloud
• Timing the Early Assembly of the Milky Way 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
• Origin of the excess of high-energy retrograde stars in the Galactic halo
• 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
• Merger-induced galaxy transformations in the ARTEMIS simulations
• 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
• Age Determination of Galaxy Merger Remnant Stars using Asteroseismology
• Influence of the Gaia-Sausage-Enceladus on the density shape of the Galactic stellar halo revealed by halo K giants from the LAMOST survey

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• 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