Understanding Satellite Galaxies Through the Kinematics of the Small Magellanic Cloud in the Age of HST and Gaia

With the advent of modern all-sky surveys, the number of known Milky Way (MW) satellite galaxies has expanded well beyond the original eleven classical satellites. One key use of this catalog of objects has been to test predictions of cosmological cold dark matter simulations on the smallest scales in the universe. However, these tests rely on modeling of the density profiles of the satellite galaxies. To build the most accurate model, three dimensional motions of the stars in the satellite are required, a measurement only now becoming possible with the new era of precision astrometry, led by work with the Hubble Space Telescope (HST) and the Gaia Space Telescope. We use data from both of these observatories to study one of the brightest nearby satellites, the Small Magellanic Cloud (SMC), to both understand its own history with the MW and its companion the Large Magellanic Cloud (LMC) and develop a novel kinematic modeling technique for application to the broader MW satellite population.

Given the SMC's irregular nature and large spatial extent across the sky, we require broad spatial sampling to thoroughly study its kinematics. To provide this, we observe 30 new fields in the SMC using HST, producing a new proper motion catalog for analysis. Using this catalog, we improve the known systemic motion of the SMC and constrain the minimum separation distance in its last interaction with the LMC to roughly 7 kpc. This places the center of the SMC passing directly through the disk of the LMC, indicating a highly turbulent interaction. Internal kinematics reveal coherent outward motion in the southeastern side of the SMC in the direction of the Magellanic Bridge, consistent with the scenario of ongoing tidal disruption.

The Gaia Data Release 2 (DR2) expanded our proper motion catalogs to well over a billion stars across the MW with proper motions, including many thousands in the direction of the Magellanic Clouds. We use this database to present the first kinematic characterization of the stellar component of the Magellinc Bridge. This analysis reveals in the young stars a roughly linearly increasing relative motion from the SMC towards the LMC, at velocities above 100 km/s, suggesting an active outflow of stars (and gas) from the SMC towards the LMC. We compare these kinematics against numerical simulations of the interactions between the SMC and LMC and find good agreement for a recent direct collision scenario, consistent with our original HST work.

However, significant uncertainties remain in our understanding of the full internal kinematics of the SMC. We present a new analysis of this system using the large DR2 catalog, attempting to account for both possible coherent rotation within the SMC and a tidal expansion component due to the LMC, as suggested by both our Bridge and earlier HST results. To capture the full 3D information present in the observations (as the proper motions are measured for stars at varying depths along the line of sight), we generate a toy 3D model of the SMC and create mock data for comparisons to the DR2 catalog. We find a need for an updated center of mass location and systemic motion for the older stellar population compared to earlier averaged measurements for the SMC, with the older stars located further from the LMC and moving away from the LMC faster than the younger stars. Taken together, we can understand this as a Bullet Cluster-like scenario where existent old stellar populations in the SMC and LMC passed through relatively unscathed while the gas violently collided, imparting different kinematics on the stars formed post-interaction. Intriguingly, we find a need for a non-zero rotation throughout much of the SMC, at a relatively high inclination angle, in addition to accounting for the tidal expansion in the SMC RGs caused by recent interactions with the LMC.

We present a similar analysis for the red supergiant (RSG) population in the SMC, whose age closely coincides with the time of last interaction between the SMC and LMC. We find compelling evidence for the existence of coherent rotation in a subpopulation of the RSGs, potentially mapping onto a previously measured age bimodality in the SMC Classical Cepheid population. The rotation signal appears when the internal kinematics are studied assuming the systemic properties for the SMC RG population, suggesting that the gas within the SMC may have been relatively unperturbed before the most recent interaction. The small area of rotation may also provide new leverage on improving the constraints on the mass ratios of the Magellanic system as it could place a hard boundary on where the gas remained fully bound to the SMC.

Taken as a whole, the analysis of the SMC has led to the creation of a modeling framework capable of accounting for compounding kinematic mechanisms (like rotation and tidal expansion) and developing intuition for understanding the broader MW satellite population in a truly 3D manner.