The Milky Way Stellar Halo Is Twisted and Doubly Broken: Insights from DESI DR2 Milky Way Survey Observation

Abstract Using K giants from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) Milky Way Survey, we measure the shape, orientation, radial profile, and density anisotropies of the Milky Way (MW) stellar halo over 8 kpc < r GC < 200 kpc. We identify a triaxial stellar halo (axis ratio 10:8:7), 43° tilted from the disk, showing two break radii at ∼16 and ∼76 kpc, likely associated with Gaia-Sausage/Enceladus and the Large Magellanic Cloud (LMC), respectively. The inner stellar halo (<30 kpc) is oblate and aligned with the disk, whereas the outer stellar halo becomes prolate and perpendicular to the disk, consistent with the vast polar structure of MW satellites. The twisted halo may arise from the disk−halo angular momentum shift triggered by the infall of a massive satellite. The anisotropic density distribution of the stellar halo is also measured, with successful reidentification of the Hercules-Aquila Cloud North/South (HAC-N/S) overdensity and the Virgo overdensity (VOD). Break radii are found at 15 and 30 kpc for VOD and HAC-N/S, respectively. We identify the LMC transient density wake with a break radius at 60 kpc in the Pisces overdensity region. We also find new observational evidence of the LMC collective density wake, by showing a break radius at ∼100 kpc in the northern Galactic cap with a clear density peak at 90 kpc. In the end, we found that more metal-poor halo stars are more radially extended. Our results provide important clues to the assembly and evolution of the MW stellar halo under the standard cosmic structure formation framework.

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Journal
The Astrophysical Journal
Published
2026-02-26
DOI
https://doi.org/10.3847/1538-4357/ae41b9
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
0.00

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article

The Milky Way Stellar Halo Is Twisted and Doubly Broken: Insights from DESI DR2 Milky Way Survey Observation

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The Astrophysical Journal
Stellar, planetary, and galactic studies
article

The Milky Way Stellar Halo Is Twisted and Doubly Broken: Insights from DESI DR2 Milky Way Survey Observation

Songting Li, Mika Lambert, Hu Zou Hu Zou, Oleg Y. Gnedin, Namitha Kizhuprakkat, Nathan R. Sandford, Leandro José Beraldo e Silva, C. Lamman, Laurent Le Guillou, Dick Joyce, Gustavo E. Medina, Peter Doel, Robert Kehoe, Davide Bianchi, Graziano Rossi, Joseph Harry Silber, Benjamin Alan Weaver, Jiaxin Han, Jaime E. Forero-Romero, Gregory Tarlé, Wenting Wang, Monica Valluri, David J. Schlegel, Carlos Frenk, Gaston Gutierrez, David Sprayberry, Ray Sharples, Enrique Gaztañaga, Ramon Miquel, Axel de la Macorra, Steven Ahlen, Eusebio Sanchez, João A. S. Amarante, Ting S. Li, Andrew P. Cooper, Will Percival, Sergey E. Koposov, Martin Landriau, Satya Gontcho A Gontcho, Anthony Kremin, Alis J. Deason, Francisco Prada, Todd Claybaugh, Raymond G. Carlberg, David Brooks, Tian Qiu, Jessica Nicole Aguilar, Ignasi Pérez-Ràfols
article en

Abstract

Abstract Using K giants from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) Milky Way Survey, we measure the shape, orientation, radial profile, and density anisotropies of the Milky Way (MW) stellar halo over 8 kpc < r GC < 200 kpc. We identify a triaxial stellar halo (axis ratio 10:8:7), 43° tilted from the disk, showing two break radii at ∼16 and ∼76 kpc, likely associated with Gaia-Sausage/Enceladus and the Large Magellanic Cloud (LMC), respectively. The inner stellar halo (<30 kpc) is oblate and aligned with the disk, whereas the outer stellar halo becomes prolate and perpendicular to the disk, consistent with the vast polar structure of MW satellites. The twisted halo may arise from the disk−halo angular momentum shift triggered by the infall of a massive satellite. The anisotropic density distribution of the stellar halo is also measured, with successful reidentification of the Hercules-Aquila Cloud North/South (HAC-N/S) overdensity and the Virgo overdensity (VOD). Break radii are found at 15 and 30 kpc for VOD and HAC-N/S, respectively. We identify the LMC transient density wake with a break radius at 60 kpc in the Pisces overdensity region. We also find new observational evidence of the LMC collective density wake, by showing a break radius at ∼100 kpc in the northern Galactic cap with a clear density peak at 90 kpc. In the end, we found that more metal-poor halo stars are more radially extended. Our results provide important clues to the assembly and evolution of the MW stellar halo under the standard cosmic structure formation framework.

The Astrophysical JournalVol. 999(1)
Boston University (US), Institució Catalana de Recerca i Estudis Avançats (ES), Universidad Complutense de Madrid (ES), Centre National de la Recherche Scientifique (FR), Fermi National Accelerator Laboratory (US), Lawrence Berkeley National Laboratory (US), University of Waterloo (CA), State Street (United States) (US), Universidad de Los Andes (CO), Southern Methodist University (US), Shanghai Jiao Tong University (CN), Perimeter Institute (CA), University of California, Santa Cruz (US), University of Toronto (CA), Institute for High Energy Physics (ES), University of Milan (IT), Durham University (GB), Joint Institute for Nuclear Astrophysics (US), Chinese Academy of Sciences (CN), University of Cambridge (GB), National Tsing Hua University (TW), University of Michigan (US), Brera Astronomical Observatory (IT), Sejong University (KR), Sorbonne Université (FR), Valongo Observatory (BR), Institut d'Estudis Espacials de Catalunya (ES), Kavli Institute for Particle Astrophysics and Cosmology (US), Laboratoire de Physique Nucléaire et de Hautes Énergies (FR), Royal Observatory (GB), National Observatory (BR), Institut National de Physique Nucléaire et de Physique des Particules (FR), Community Science and Data Center (US), NSF NOIRLab (US), Instituto de Astrofísica de Andalucía (ES), National Astronomical Observatories (CN), Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (ES), University College London (GB), UK Astronomy Technology Centre (GB), University of Virginia (US), The Ohio State University (US), University of Portsmouth (GB), Universidad Nacional Autónoma de México (MX), Universitat Politècnica de Catalunya (ES)
National Natural Science Foundation of China, Higher Education Discipline Innovation Project, Office of Science
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Stellar, planetary, and galactic studies
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