Red giants in the Small Magellanic Cloud – I. Disc and tidal stream kinematics

Abstract

We present results from an extensive spectroscopic survey of field stars in the Small Magellanic Cloud (SMC). 3037 sources, predominantly first-ascent red giants, spread across roughly 37.5 deg2, are analysed. The line-of-sight velocity field is dominated by the projection of the orbital motion of the SMC around the Large Magellanic Cloud/Milky Way. The residuals are inconsistent with both a non-rotating spheroid and a nearly face on disc system. The current sample and previous stellar and H i kinematics can be reconciled by rotating disc models with line-of-nodes position angle Θ ≈ 120°–130°, moderate inclination (25°–70°), and rotation curves rising at 20–40 km s−1 kpc−1. The metal-poor stars exhibit a lower velocity gradient and higher velocity dispersion than the metal-rich stars. If our interpretation of the velocity patterns as bulk rotation is appropriate, then some revision to simulations of the SMC orbit is required since these are generally tuned to the SMC disc line of nodes lying in a north-east–south-west (SW) direction. Residuals show strong spatial structure indicative of non-circular motions that increase in importance with increasing distance from the SMC centre. Kinematic substructure in the north-west part of our survey area is associated with the tidal tail or Counter-Bridge predicted by simulations. Lower line-of-sight velocities towards the Wing and the larger velocities just beyond the SW end of the SMC Bar are probably associated with stellar components of the Magellanic-Bridge and Counter-Bridge, respectively. Our results reinforce the notion that the intermediate-age stellar population of the SMC is subject to substantial stripping by external forces.