Deciphering the Kinematic Structure of the Small Magellanic Cloud
  through its Red Giant Population
  We present a new kinematic model for the Small Magellanic Cloud (SMC), using
data from the \gaia\ Data Release 2 catalog. We identify a sample of
astrometrically well-behaved red giant (RG) stars belonging to the SMC and
cross-match with publicly available radial velocity (RV) catalogs. We create a
3D spatial model for the RGs, using RR Lyrae for distance distributions, and
apply kinematic models with varying rotation properties and a novel tidal
expansion prescription to generate mock proper motion (PM) catalogs. When we
compare this series of mock catalogs to the observed RG data, we find a
combination of moderate rotation (with a magnitude of $\sim10-20$ km s$^{-1}$
at 1 kpc from the SMC center, inclination between $\sim50-80$ degrees, and a
predominantly north-to-south line of nodes position angle of $\sim180$ degrees)
and tidal expansion (with a scaling of $\sim10$ km s$^{-1}$ kpc$^{-1}$) is
required to explain the PM signatures. The exact best-fit parameters depend
somewhat on whether we assess only the PMs or include the RVs as a qualitative
check, leaving some small tension remaining between the PM and RV conclusions.
In either case, the parameter space preferred by our model is different both
from previously inferred rotational geometries, including from the SMC H{\small
I} gas and from the RG RV-only analyses, and new SMC PM analyses which conclude
that a rotation signature is not detectable. Taken together this underscores
the need to treat the SMC as a series of different populations with distinct
kinematics.
