The Proper Motion Field of the Small Magellanic Cloud: Kinematic Evidence for its Tidal Disruption

TL;DR

This study measures the proper motion of the Small Magellanic Cloud using HST and Gaia data, revealing signs of tidal disruption and a recent collision with the Large Magellanic Cloud approximately 147 million years ago.

Contribution

It provides the most precise systemic proper motion measurement of the SMC and evidence for its tidal disruption and recent interaction with the LMC.

Findings

SMC shows little rotation after systemic motion subtraction.

Outer regions of SMC exhibit radial motion indicating tidal disruption.

Most interactions with LMC occurred about 147 million years ago.

Abstract

We present a new measurement of the systemic proper motion of the Small Magellanic Cloud (SMC), based on an expanded set of 30 fields containing background quasars and spanning a $\sim$3 year baseline, using the \textit{Hubble Space Telescope} (\textit{HST}) Wide Field Camera 3. Combining this data with our previous 5 \textit{HST} fields, and an additional 8 measurements from the \textit{Gaia}-Tycho Astrometric Solution Catalog, brings us to a total of 43 SMC fields. We measure a systemic motion of $\mu_{W}$ = $-0.82$ $\pm$ 0.02 (random) $\pm$ 0.10 (systematic) mas yr$^{-1}$ and $\mu_{N}$ = $-1.21$ $\pm$ 0.01 (random) $\pm$ 0.03 (systematic) mas yr$^{-1}$. After subtraction of the systemic motion, we find little evidence for rotation, but find an ordered mean motion radially away from the SMC in the outer regions of the galaxy, indicating that the SMC is in the process of tidal disruption. We model the past interactions of the Clouds with each other based on the measured present-day relative velocity between them of $103 \pm 26$ km s$^{-1}$. We find that in 97\% of our considered cases, the Clouds experienced a direct collision $147 \pm 33$ Myr ago, with a mean impact parameter of $7.5 \pm 2.5$ kpc.