A re-interpretation of the Triangulum-Andromeda stellar clouds: a population of halo stars kicked out of the Galactic disk

Abstract

The Triangulum-Andromeda stellar clouds (TriAnd1 and TriAnd2) are a pair of concentric ring- or shell-like over-densities at large $R$ ($\approx$ 30 kpc) and $Z$ ($\approx$ -10 kpc) in the Galactic halo that are thought to have been formed from the accretion and disruption of a satellite galaxy. This paper critically re-examines this formation scenario by comparing the number ratio of RR Lyrae to M giant stars associated with the TriAnd clouds with other structures in the Galaxy. The current data suggest a stellar population for these over-densities ($f_{\mathrm{RR}:\mathrm{MG}}<0.38$ at 95% confidence) quite unlike any of the known satellites of the Milky Way ($f_{\mathrm{RR}:\mathrm{MG}}\approx 0.5$ for the very largest and $f_{\mathrm{RR}:\mathrm{MG}}>>1$ for the smaller satellites) and more like the population of stars born in the much deeper potential well inhabited by the Galactic disk ($f_{\mathrm{RR}:\mathrm{MG}}<0.01$). N-body simulations of a Milky-Way-like galaxy perturbed by the impact of a dwarf galaxy demonstrate that, in the right circumstances, concentric rings propagating outwards from that Galactic disk can plausibly produce similar over-densities. These results provide dramatic support for the recent proposal by Xu et al. (2015) that, rather than stars accreted from other galaxies, the TriAnd clouds could represent stars kicked-out from our own disk. If so, these would be the first populations of disk stars to be found in the Galactic halo and a clear signature of the importance of this second formation mechanism for stellar halos more generally. Moreover, their existence at the very extremities of the disk places strong constraints on the nature of the interaction that formed them.

27 pages, 8 figures; published in MNRAS

References in corpus (15)

• The Haunted Halos of Andromeda and Triangulum: A panorama of galaxy formation in action
• The Dual Origin of Stellar Halos
• Rings and Radial Waves in the Disk of the Milky Way
• A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: Variation of the Metallicity Distribution Function Along the Sagittarius Stream
• The Chemical Enrichment History of the Large Magellanic Cloud
• The PAndAS Field of Streams: stellar structures in the Milky Way halo toward Andromeda and Triangulum
• The Nature and Origin of Substructure in the Outskirts of M31. I. Surveying the Stellar Content with HST/ACS
• The Nature and Origin of Substructure in the Outskirts of M31 -- II. Detailed Star Formation Histories
• Galactic halo stellar structures in the Triangulum-Andromeda region
• Evidence for an Overluminosity of the Variable Star RR Lyr, and a Revised Distance to the LMC
• Characterizing the High-Velocity Stars of RAVE: The Discovery of a Metal-Rich Halo Star Born in the Galactic Disk
• Identifying Contributions to the Stellar Halo from Accreted, Kicked-Out, and In Situ Populations
• Exploring Halo Substructure with Giant Stars: The Nature of the Triangulum-Andromeda Stellar Features
• Does SEGUE/SDSS indicate a dual Galactic halo?
• Clustered Cepheid Variables 90 kiloparsec from the Galactic Center

Cited by in corpus (76)

• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• Target Selection for the SDSS-IV APOGEE-2 Survey
• Footprints of the Sagittarius dwarf galaxy in the $Gaia$ data set
• The Influence of Sagittarius and the Large Magellanic Cloud on the Milky Way Galaxy
• Response of the Milky Way's disc to the Large Magellanic Cloud in a first infall scenario
• The origin of galactic metal-rich stellar halo components with highly eccentric orbits
• Warp, Waves, and Wrinkles in the Milky Way
• Chemo-kinematics of the $Gaia$ RR Lyrae: the halo and the disc
• Warps and waves in the stellar discs of the Auriga cosmological simulations
• The Canada-France Imaging Survey: First results from the u-band component
• Final Targeting Strategy for the SDSS-IV APOGEE-2N Survey
• Gaia Early Data Release 3: The Galactic anticentre
• A fully cosmological model of a Monoceros-like ring
• Galactic Archaeology with Gaia
• Two chemically similar stellar overdensities on opposite sides of the plane of the Galaxy
• Hidden depths in the local Universe: The Stellar Stream Legacy Survey
• Common origin for Hercules-Aquila and Virgo Clouds in Gaia DR2
• An all-sky proper motion map of the Sagittarius stream using Gaia DR2
• Kinematics with Gaia DR2: The Force of a Dwarf
• Age Demographics of the Milky Way Disk and Bulge
• A missing outskirts problem? Comparisons between stellar halos in the Dragonfly Nearby Galaxies Survey and the TNG100 simulation
• Tails and streams around the Galactic globular clusters NGC1851, NGC1904, NGC2298 and NGC2808
• The Galactic Thick Disc density profile traced with RR Lyrae stars
• Chemo-dynamical properties of the Anticenter Stream: a surviving disc fossil from a past satellite interaction
• Unmixing the Galactic Halo with RR Lyrae tagging
• Weighing the Galactic disk using phase-space spirals II. Most stringent constraints to a thin dark disk using Gaia EDR3
• Mapping the Monoceros Ring in 3D with Pan-STARRS1
• A Disk Origin for the Monoceros Ring and A13 Stellar Overdensities
• Elemental Abundances in M31: A Comparative Analysis of Iron and Alpha Element Abundances in the Outer Disk, Giant Stellar Stream, and Inner Halo of M31
• Kinematics of the Palomar 5 stellar stream from RR Lyrae stars
• Discovery of a Stellar Overdensity in Eridanus-Phoenix in the Dark Energy Survey
• Phase Wrapping of Epicyclic Perturbations in the Wobbly Galaxy
• Exploring the pertubed Milky Way disk and the substructures of the outer disk
• Milky Way Tomography with K and M Dwarf Stars: the Vertical Structure of the Galactic Disk
• Exploring Halo Substructure with Giant Stars. XV. Discovery of a Connection between the Monoceros Ring and the Triangulum-Andromeda Overdensity?
• HALO7D II: The Halo Velocity Ellipsoid and Velocity Anisotropy with Distant Main Sequence Stars
• Stellar disc streams as probes of the Galactic potential and satellite impacts
• A Comprehensive Perturbative Formalism for Phase-Mixing in Perturbed Disks. I. Phase spirals in an Infinite, Isothermal Slab
• Orbital Torus Imaging: Using Element Abundances to Map Orbits and Mass in the Milky Way
• Discovery of a disrupting open cluster far into the Milky Way halo: a recent star formation event in the leading arm of the Magellanic stream?
• Measuring the vertical response of the Galactic disc to an infalling satellite
• Snails Across Scales: Local and Global Phase-Mixing Structures as Probes of the Past and Future Milky Way
• Weighing the Galactic disk using phase-space spirals I. Tests on one-dimensional simulations
• Disk-Like Chemistry of the Triangulum-Andromeda Overdensity as Seen by APOGEE
• Disentangling the Virgo Overdensity with RR Lyrae stars
• A-type stars in the Canada-France Imaging Survey II. Tracing the height of the disc at large distances with Blue Stragglers
• HALO7D I: The Line of Sight Velocities of Distant Main Sequence Stars in the Milky Way Halo
• The Great Wave: Evidence of a large-scale vertical corrugation propagating outwards in the Galactic disc
• Kinematics beats dust: unveiling nested substructure in the perturbed outer disc of the Milky Way
• The metal-rich halo component extended in z: a characterization with Gaia DR2 and APOGEE
• The outer disc in shambles: blind detection of Monoceros and ACS with Gaia's astrometric sample
• Eigenfunctions of Galactic Phase Space Spirals from Dynamic Mode Decomposition
• The Milky Way, Coming into Focus: Precision Astrometry Probes its Evolution, and its Dark Matter
• Estimating the local dark matter density in a non-axisymmetric wobbling disc
• A Map of the Local Velocity Substructure in the Milky Way Disk
• Beyond Gaia: Asteroseismic Distances of M giants using Ground-Based Transient Surveys
• The Implications of Local Fluctuations in the Galactic Midplane for Dynamical Analysis in the Gaia Era
• Blue extreme disk-runaway stars with Gaia EDR3
• The Dragonfly Edge-on Galaxies Survey: Shaping the outer disc of NGC 4565 via accretion
• A Holistic Review of a Galactic Interaction
• Spectroscopic Follow-Up of the Hercules Aquila Cloud
• Frequencies, chaos and resonances: a study of orbital parameters of nearby thick disc and halo stars
• The Panchromatic Hubble Andromeda Treasury XX: The Disk of M31 is Thick
• Cartography of Triangulum-Andromeda using SDSS stars
• Beyond the Local Volume. II. Population Scaleheights and Ages of Ultracool Dwarfs in Deep HST/WFC3 Parallel Fields
• Applying Noether's theorem to matter in the Milky Way: evidence for external perturbations and non-steady-state effects from Gaia Data Release 2
• Can Stellar Discs in a Cosmological Setting Avoid Forming Strong Bars?
• Evidence That Hydra I is a Tidally Disrupting Milky Way Dwarf Galaxy
• Characterizing the SHARDS of Disrupted Milky Way Satellites with LAMOST
• Triangulum-Andromeda overdensity: a region with a complex stellar population
• Two-point correlation function studies for the Milky Way: discovery of spatial clustering from disk excitations and substructure
• The Chemodynamical Nature of the Triangulum-Andromeda Overdensity
• Detailed Mapping of the Galactic Disk Structure in the Solar Neighborhood through LAMOST K Dwarfs
• The Vertical Displacement of the Milky Way Disk
• The Multidimensional Milky Way
• Bending Waves in Velocity Space: a First Look at the THINGS sample