DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor

Abstract

We present DESI observations of the inner halo of M31, which reveal the kinematics of a recent merger - a galactic immigration event - in exquisite detail. Of the 11,416 sources studied in 3.75 hour of on-sky exposure time, 7,438 are M31 sources with well measured radial velocities. The observations reveal intricate coherent kinematic structure in the positions and velocities of individual stars: streams, wedges, and chevrons. While hints of coherent structures have been previously detected in M31, this is the first time they have been seen with such detail and clarity in a galaxy beyond the Milky Way. We find clear kinematic evidence for shell structures in the Giant Stellar Stream, the Northeast Shelf and Western Shelf regions. The kinematics are remarkably similar to the predictions of dynamical models constructed to explain the spatial morphology of the inner halo. The results are consistent with the interpretation that much of the substructure in the inner halo of M31 is produced by a single galactic immigration event 1 - 2 Gyr ago. Significant numbers of metal-rich stars ([Fe/H]$>-0.5$) are present in all of the detected substructures, suggesting that the immigrating galaxy had an extended star formation history. We also investigate the ability of the shells and Giant Stellar Stream to constrain the gravitational potential of M31, and estimate the mass within a projected radius of 125 kpc to be $\mathrm{lo}{\mathrm{g}}_{10}{M}_{\mathrm{NFW}}(<125\mathrm{kpc})/M_{\odot }=11.80_{-0.10}^{+0.12}$. The results herald a new era in our ability to study stars on a galactic scale and the immigration histories of galaxies.

45 pages, 22 figures, 8 tables; Astrophysical Journal in press; Data at https://zenodo.org/record/6977494

References in corpus (43)

• The Gaia mission
• Overview of the Instrumentation for the Dark Energy Spectroscopic Instrument
• The accretion origin of the Milky Way's stellar halo
• The Haunted Halos of Andromeda and Triangulum: A panorama of galaxy formation in action
• The unWISE Catalog: Two Billion Infrared Sources from Five Years of WISE Imaging
• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• unWISE: unblurred coadds of the WISE imaging
• The CatWISE2020 Catalog
• The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass
• Kraken reveals itself -- the merger history of the Milky Way reconstructed with the E-MOSAICS simulations
• A time-resolved picture of our Milky Way's early formation history
• Galactic Angular Momentum in the Illustris Simulation: Feedback and the Hubble Sequence
• The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31)
• A deep kinematic survey of planetary nebulae in the Andromeda Galaxy using the Planetary Nebula Spectrograph
• The Target-selection Pipeline for the Dark Energy Spectroscopic Instrument
• Investigating the Andromeda Stream: III. A Young Shell System in M31
• The Panchromatic Hubble Andromeda Treasury XI: The Spatially-Resolved Recent Star Formation History of M31
• Timing the Early Assembly of the Milky Way with the H3 Survey
• Deep Full-sky Coadds from Three Years of WISE and NEOWISE Observations
• An updated survey of globular clusters in M31. II Newly discovered bright and remote clusters
• Global Properties of M31's Stellar Halo from the SPLASH Survey: II. Metallicity Profile
• Was the Andromeda Stream Produced by a Disk Galaxy?
• A clear age-velocity dispersion correlation in Andromeda's stellar disk
• The Need for Speed: Escape velocity and dynamical mass measurements of the Andromeda galaxy
• Stellar Kinematics in the Complicated Inner Spheroid of M31: Discovery of Substructure Along the Southeastern Minor Axis and its Relationship to the Giant Southern Stream
• The ANTARES Astronomical Time-Domain Event Broker
• Elemental Abundances in M31: The Kinematics and Chemical Evolution of Dwarf Spheroidal Satellite Galaxies
• The Once and Future Andromeda Stream
• Energy wrinkles and phase-space folds of the last major merger
• Two major accretion epochs in M31 from two distinct populations of globular clusters
• The infall of dwarf satellite galaxies are influenced by their host's massive accretions
• The survey of planetary nebulae in Andromeda (M 31) V. Chemical enrichment of the thin and thicker discs of Andromeda. Oxygen to argon abundance ratios for planetary nebulae and H II regions
• Elemental Abundances in M31: Gradients in the Giant Stellar Stream
• Elemental Abundances in M31: Properties of the Inner Stellar Halo
• The discreteness-driven relaxation of collisionless gravitating systems: entropy evolution and the Nyquist-Shannon theorem
• A 6-d View of Stellar Shells
• Elemental Abundances in M31: Iron and Alpha Element Abundances in M31's Outer Halo
• Estimating ages and metallicities of M31 star clusters from LAMOST DR6
• Metallicity distribution of the progenitor of the Giant Stellar Stream in the Andromeda Galaxy
• Kinematics and Metallicity of Red Giant Branch Stars in the Northeast Shelf of M31
• A Detection of Gas Associated with the M 31 Stellar Stream
• Flattening of dark matter cusps during mergers: model of M31
• The survey of Planetary Nebulae in Andromeda (M31): I. Imaging the disk and halo with MegaCam@CFHT

Cited by in corpus (23)

• The Early Data Release of the Dark Energy Spectroscopic Instrument
• Overview of the DESI Milky Way Survey
• DESI Early Data Release Milky Way Survey Value-Added Catalogue
• Structure, Kinematics, and Observability of the Large Magellanic Cloud's Dynamical Friction Wake in Cold vs. Fuzzy Dark Matter
• Evidence for a Massive Andromeda Galaxy Using Satellite Galaxy Proper Motions
• Exploring the diversity and similarity of radially anisotropic Milky Way-like stellar haloes: implications for disrupted dwarf galaxy searches
• Exploring the interaction between the MW and LMC with a large sample of blue horizontal branch stars from the DESI survey
• The survey of planetary nebulae in Andromeda (M31) VI. Kinematics of M31 inner-halo substructures and comparison with major-merger simulation predictions
• Elemental abundances in M31: Individual and Coadded Spectroscopic [Fe/H] and [α/Fe] throughout the M31 Halo with SPLASH
• The Non-Axisymmetric Influence: Radius and Angle-Dependent Trends in a Barred Galaxy
• Prospects for Detecting Gaps in Globular Cluster Stellar Streams in External Galaxies with the Nancy Grace Roman Space Telescope
• Investigating the Dark Matter Halo of NGC 5128 using a Discrete Dynamical Model
• Globular Cluster Counts around 700 Nearby Galaxies
• The survey of planetary nebulae in Andromeda (M31) VII. Predictions of a major merger simulation model compared with chemodynamical data of the disc and inner halo substructures
• Dark matter fraction derived from the M31 rotation curve
• The origin of the metallicity distributions of the NE and W stellar shelves in the Andromeda Galaxy
• First resolved stellar halo kinematics of a MW-mass galaxy outside the Local Group: A flat counter-rotating halo in NGC 4945
• One Stream or Two -- Exploring Andromeda's North West Stream
• Resonant Effects of a Bar on the Galactic Disk Kinematics Perpendicular to Its Plane
• Timing Mass of the Local Group
• A Pearl in the Shell: an ultra-compact dwarf within the tidal debris surrounding spiral galaxy NGC 7531
• Properties of the Lower Segment of M31's North West Stream
• Quasars behind the disk of M31 galaxy