2026-03-16 アリゾナ大学
Illustration of the SMC-LMC collision.Image courtesy of Himansh Rathore, Steward Observatory
＜関連情報＞
- https://news.arizona.edu/news/galaxy-next-door-transforming-and-astronomers-can-see-it-happening
- https://iopscience.iop.org/article/10.3847/1538-4357/ae4507
- https://iopscience.iop.org/article/10.3847/1538-4357/ade0ae/meta
銀河の変容―小マゼラン雲の構造的および運動学的不均衡の理解 A Galactic Transformation—Understanding the SMC’s Structural and Kinematic Disequilibrium
Himansh Rathore, Gurtina Besla, Roeland P. van der Marel, and Nitya Kallivayalil
The Astrophysical Journal Published: 2026 March 16
DOI:10.3847/1538-4357/ae4507
Abstract
The SMC is in disequilibrium. Gas line-of-sight (LOS) velocity maps show a gradient of 60–100 km s−1, generally interpreted as a rotating gas disk consistent with the Tully–Fisher relation. Yet, the stars don’t show rotation. Despite a small on-sky extent (∼4 kpc), the SMC exhibits a large (∼10 kpc) LOS depth, and the stellar photometric center is offset from the HI kinematic center by ∼1 kpc. With N-body hydrodynamical simulations, we show that a recent (∼100 Myr ago) SMC–LMC collision (impact parameter ∼2 kpc) explains the observed SMC’s internal structure and kinematics. The simulated SMC is initialized with rotating stellar and gaseous disks. Post-collision, the SMC’s tidal tail accounts for the large LOS depth. The SMC’s stellar kinematics become dispersion dominated (v/σ ≈ 0.2), with radially outward motions at R > 2 kpc, and a small (<10 km s−1) remnant rotation at R < 2 kpc, consistent with observations. Post-collision gas kinematics are also dominated by radially outward motions, without remnant rotation. Hence, the observed SMC’s gas LOS velocity gradient is due to radial motions as opposed to disk rotation. Ram pressure from the LMC’s gas disk during the collision imparts ≈30 km s−1 kick to the SMC’s gas, sufficient to destroy gas rotation and offset the SMC’s stellar and gas centers. Our work highlights the critical role of group processing through galaxy collisions in driving dIrr to dE/dSph transformation, including the removal of gas. Consequently, frameworks that treat the SMC as a galaxy in transformation are required to effectively use its observational data to constrain interstellar medium and dark matter physics.
LMCのバーが最近のSMC衝突に反応し、SMCのダークマタープロファイルに及ぼす影響 Response of the LMC’s Bar to a Recent SMC Collision and Implications for the SMC’s Dark Matter Profile
Himansh Rathore, Gurtina Besla, Kathryne J. Daniel, and Leandro Beraldo e Silva
The Astrophysical Journal Published: 2025 July 15
DOI:10.3847/1538-4357/ade0ae
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. Using N-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) × 109 M⊙. Therefore, the LMC bar’s tilt constrains the SMC’s pre-collision DM profile, and requires the SMC to be a DM-dominated galaxy.
