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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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Origins of the Evil Eye: M64's Stellar Halo Reveals the Recent Accretion of an SMC-mass Satellite
Abstract M64, often called the “Evil Eye” galaxy, is unique among local galaxies. Beyond its dramatic, dusty nucleus, it also hosts an outer gas disk that counter-rotates relative to its stars. The mass of this outer disk is comparable to the gas content of the Small Magellanic Cloud (SMC), prompting the idea that it was likely accreted in a recent minor merger. Yet, detailed follow-up studies of M64's outer disk have shown no evidence of such an event, leading to other interpretations, such as a “flyby” interaction with the distant diffuse satellite Coma P. We present Subaru Hyper Suprime-Cam observations of M64's stellar halo, which resolve its stellar populations and reveal a spectacular radial shell feature, oriented ∼30° relative to the major axis and along the rotation axis of the outer gas disk. The shell is ∼45 kpc southeast of M64, while a similar but more diffuse plume to the northwest extends to >100 kpc. We estimate a stellar mass and metallicity for the southern shell ofM⋆= 1.80 ± 0.54 × 108M⊙and [M/H] = −1.0, respectively, and a similar mass of 1.42 ± 0.71 × 108M⊙for the northern plume. Taking into account the accreted material in M64's inner disk, we estimate a total stellar mass for the progenitor satellite ofM⋆,prog≃ 5 × 108M⊙. These results suggest that M64 is in the final stages of a minor merger with a gas-rich satellite strikingly similar to the SMC, in which M64's accreted counter-rotating gas originated, and which is responsible for the formation of its dusty inner star-forming disk.
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- Award ID(s):
- 2007065
- PAR ID:
- 10419718
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 949
- Issue:
- 2
- ISSN:
- 2041-8205
- Format(s):
- Medium: X Size: Article No. L37
- Size(s):
- Article No. L37
- Sponsoring Org:
- National Science Foundation
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