A Galactic Transformation—Understanding the SMC’s Structural and Kinematic Disequilibrium

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.