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 ($\sim4$ kpc), the SMC exhibits a large ($\sim10$ kpc) LoS depth, and the stellar photometric center is offset from the HI kinematic center by $\sim$1 kpc. With N-body hydrodynamical simulations, we show that a recent ($\sim$100 Myr ago) SMC-LMC collision (impact parameter $\sim2$ 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 ($v/σ\approx0.2$), 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 $\approx30$ 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.

💡 Research Summary

This paper addresses the long‑standing puzzle of the Small Magellanic Cloud (SMC): its neutral‑hydrogen (HI) gas exhibits a line‑of‑sight (LoS) velocity gradient of 60–100 km s⁻¹ that has traditionally been interpreted as rotation of a gas disk, yet its stellar component shows virtually no rotation and is dominated by dispersion. Moreover, the SMC has a large depth along the line of sight (5–20 kpc) despite a modest on‑sky size (~4 kpc), and the photometric centre of the stars is offset from the HI kinematic centre by about 1 kpc. The authors propose that a recent (∼100 Myr ago) direct collision with the Large Magellanic Cloud (LMC), with an impact parameter of ~2 kpc, can simultaneously explain all these anomalies.

To test this hypothesis they employ the Besla et al. (2012) “B12” suite of high‑resolution N‑body + smoothed‑particle‑hydrodynamics (SPH) simulations, which model the LMC, SMC, and the Milky Way (MW) halo. Two scenarios are examined: Model 1 (no close encounter, minimum separation ≈30 kpc) and Model 2 (a direct LMC‑SMC collision 100 Myr ago). Both galaxies are initialized with live Hernquist dark‑matter halos, exponential stellar disks, and SPH gas disks. The SMC is set up to lie on the Baryonic Tully‑Fisher Relation (BTFR) with a peak rotation of ~60 km s⁻¹ and a total baryonic mass of ~1.05×10⁹ M⊙. The simulations are run with Gadget‑3, using a sub‑grid multiphase ISM model, radiative cooling, and star formation prescriptions.

The results from Model 2 are striking. The collision injects a ~30 km s⁻¹ ram‑pressure “kick” to the SMC’s gas as it ploughs through the LMC’s gaseous disk. This ram pressure, together with strong tidal forces, destroys the pre‑existing gas rotation. Post‑collision the gas exhibits predominantly radial outward motions; the observed LoS velocity gradient is therefore a projection of this radial flow rather than true rotation. The gas retains only a weak residual rotation (<10 km s⁻¹) inside 2 kpc, consistent with HI observations that show a modest central gradient.

Stellar dynamics respond similarly. The stellar disk is torn apart, forming an extended tidal tail that accounts for the large LoS depth. The stellar kinematics become dispersion‑dominated with v/σ≈0.2. Inside 2 kpc a faint rotation (<10 km s⁻¹) persists, but beyond this radius the stars move outward radially, reproducing the observed depth and the lack of a coherent rotation curve. The simulation also naturally reproduces the ∼1 kpc offset between the stellar photometric centre and the HI kinematic centre, a direct consequence of the asymmetric tidal stripping and ram‑pressure displacement.

These findings have broader implications for dwarf galaxy evolution. The authors argue that the SMC is undergoing a transformation from a gas‑rich dwarf irregular (dIrr) to a dwarf elliptical/spheroidal (dE/dSph) driven by group‑scale processing—specifically, a close galaxy‑galaxy collision. The collision simultaneously removes angular momentum from the gas, quenches rotation, and creates a dispersion‑supported stellar system, while also generating large line‑of‑sight depth and centre offsets. Consequently, any attempt to infer the SMC’s dark‑matter halo properties using equilibrium‑based methods (e.g., Jeans modeling) would be severely biased. Instead, non‑equilibrium dynamical modeling, as demonstrated here, is required to extract reliable mass profiles.

The paper also discusses limitations: standard SPH underestimates ram‑pressure forces, star‑formation feedback is treated in a simplified manner, and the static Milky Way potential ignores possible dynamical response. The authors suggest future work with higher‑resolution magnetohydrodynamic simulations and deeper observational campaigns (e.g., Gaia EDR3 proper motions, ASKAP HI mapping) to refine the collision timeline and quantify the gas stripping efficiency.

In conclusion, the study provides compelling evidence that a recent direct LMC‑SMC collision can account for the SMC’s structural and kinematic disequilibrium. It highlights the pivotal role of galaxy collisions in dwarf galaxy transformation and underscores the necessity of treating the SMC as a non‑equilibrium system when using it as a laboratory for interstellar medium physics, star formation, and dark‑matter studies.