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Abstract We present new measurements of the 2D geometry of the LMC’s stellar bar with precise astrometric observations of red clump stars in Gaia DR3. We develop a novel solution to tackle crowding-induced incompleteness in Gaia data sets with the Gaia BP-RP color excess. Utilizing the color excess information, we derive a 2D completeness map of the LMC’s disk. We find that incompleteness biases the bar measurements and induces large uncertainties. With the completeness-corrected 2D red clump map, we precisely measure the LMC bar’s properties using Fourier decomposition. The bar radius (semimajor axis) is kpc, and its position angle is 121.°26 ± 0.°21. The bar’s strength as quantified by the Fourier bi-symmetric amplitude isSbar= 0.27, indicating that the LMC has a significant bar perturbation. We find the bar has an axis ratio of 0.54 ± 0.03, and is offset with respect to the center of the outer disk isophote atR≈ 5 kpc by 0.76 ± 0.01 kpc. These LMC bar properties agree with a hydrodynamic model where the SMC has undergone a recent direct collision with the LMC. We compare the LMC’s bar properties with other barred galaxies in the local Universe, and discover that the LMC is similar to other barred galaxies in terms of bar-galaxy scaling relations. We discuss how our completeness correction framework can be applied to other systems in the Local Group.
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This content will become publicly available on July 15, 2026
Response of the LMC’s Bar to a Recent SMC Collision and Implications for the SMC’s Dark Matter Profile
Abstract The LMC’s stellar bar is offset from the outer disk center, tilted from the disk plane, and does not drive gas inflows. These properties are atypical of bars in gas-rich galaxies, yet the LMC bar’s strength and radius are similar to typical barred galaxies. UsingN-body hydrodynamic simulations, we show that the LMC’s unusual bar is explainable if there was a recent collision (impact parameter ≈2 kpc) between the LMC and SMC. Pre-collision, the simulated bar is centered and coplanar. Post-collision, the simulated bar is offset (≈1.5 kpc) and tilted (≈8 6). The simulated bar offset reduces with time, and comparing with the observed offset (≈0.8 kpc) suggests the timing of the true collision to be 150–200 Myr ago. Then, 150 Myr post-collision, the LMC’s bar is centered with its dark matter (DM) halo, whereas the outer disk center is separated from the DM center by ≈1 kpc. The SMC collision produces a tilted-ring structure for the simulated LMC, consistent with observations. Post-collision, the simulated LMC bar’s pattern speed decreases by a factor of 2. We also provide a generalizable framework to quantitatively compare the LMC’s central gas distribution in different LMC–SMC interaction scenarios. We demonstrate that the SMC’s torques on the LMC’s bar during the collision are sufficient to explain the observed bar tilt, provided the SMC’s total mass within 2 kpc was (0.8–2.4) × 109M⊙. Therefore, the LMC bar’s tilt constrains the SMC’s pre-collision DM profile, and requires the SMC to be a DM-dominated galaxy.
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- Award ID(s):
- 1941096
- PAR ID:
- 10624087
- Publisher / Repository:
- American Astronomical Society
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 988
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 79
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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