Deciphering the Kinematic Structure of the Small Magellanic Cloud through its Red Giant Population

Abstract

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 $\sim 10-20$ km s${}^{-1}$ at 1 kpc from the SMC center, inclination between $\sim 50-80$ degrees, and a predominantly north-to-south line of nodes position angle of $\sim 180$ degrees) and tidal expansion (with a scaling of $\sim 10$ 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.

17 pages, 11 figures, 2 tables, submitted to ApJ

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