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