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Abstract The shape and orientation of dark matter (DM) halos are sensitive to the microphysics of the DM particles, yet in many mass models, the symmetry axes of the Milky Way’s DM halo are often assumed to be aligned with the symmetry axes of the stellar disk. This is well motivated for the inner DM halo, but not for the outer halo. We use zoomed-in cosmological baryonic simulations from the Latte suite of FIRE-2 Milky Way–mass galaxies to explore the evolution of the DM halo’s orientation with radius and time, with or without a major merger with a Large Magellanic Cloud analog, and when varying the DM model. In three of the four cold DM halos we examine, the orientation of the halo minor axis diverges from the stellar disk vector by more than 20° beyond about 30 galactocentric kpc, reaching a maximum of 30°–90°, depending on the individual halo’s formation history. In identical simulations using a model of self-interacting DM withσ= 1 cm2g−1, the halo remains aligned with the stellar disk out to ∼200–400 kpc. Interactions with massive satellites (M≳ 4 × 1010M⊙at pericenter;M≳ 3.3 × 1010M⊙at infall) affect the orientation of the halo significantly, aligning the halo’s major axis with the satellite galaxy from the disk to the virial radius. The relative orientation of the halo and disk beyond 30 kpc is a potential diagnostic of self-interacting DM, if the effects of massive satellites can be accounted for.
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This content will become publicly available on March 21, 2026
Cosmological Predictions for Minor Axis Stellar Density Profiles in the Inner Regions of Milky Way–mass Galaxies
Abstract ΛCDM cosmology predicts the hierarchical formation of galaxies, which build up mass by merger events and accreting smaller systems. The stellar halo of the Milky Way (MW) has proven to be useful a tool for tracing this accretion history. However, most of this work has focused on the outer halo where dynamical times are large and the dynamical properties of accreted systems are preserved. In this work, we investigate the inner galaxy regime, where dynamical times are relatively small and systems are generally completely phase mixed. Using the FIRE-2 and Auriga cosmological zoom-in simulation suites of MW-mass galaxies, we find the stellar density profiles along the minor axis (perpendicular to the galactic disk) within the Navarro–Frenk–White scale radii (R ≈ 15 kpc) are best described as an exponential disk with scale height < 0.3 kpc and a power-law component with slopeα ≈ −4. The stellar density amplitude and slope for the power-law component are not significantly correlated with metrics of the galaxy’s accretion history. Instead, we find the stellar profiles strongly correlate with the dark matter profile. Across simulation suites, the galaxies studied in this work have a stellar-to-dark-matter mass ratio that decreases as 1/r2along the minor axis.
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- Award ID(s):
- 2303831
- PAR ID:
- 10590890
- Publisher / Repository:
- The Astrophysical Journal
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 982
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 87
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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