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\nthe Small Magellanic Cloud (SMC). 3037 sources, predominantly first-ascent red\ngiants, spread across roughly 37.5 sq. deg, are analysed. The line of sight\nvelocity field is dominated by the projection of the orbital motion of the SMC\naround the LMC/Milky Way. The residuals are inconsistent with both a\nnon-rotating spheroid and a nearly face on disk system. The current sample and\nprevious stellar and HI kinematics can be reconciled by rotating disk models\nwith line of nodes position angle, theta, ~ 120-130 deg., moderate inclination\n(i ~ 25-70 deg.), and rotation curves rising at 20-40 km/s/kpc. The metal-poor\nstars exhibit a lower velocity gradient and higher velocity dispersion than the\nmetal-rich stars. If our interpretation of the velocity patterns as bulk\nrotation is appropriate, then some revision to simulations of the SMC orbit is\nrequired since these are generally tuned to the SMC disk line-of-nodes lying in\na NE-SW direction. Residuals show strong spatial structure indicative of\nnon-circular motions that increase in importance with increasing distance from\nthe SMC centre. Kinematic substructure in the north-west part of our survey\narea is associated with the tidal tail or Counter-Bridge predicted by\nsimulations. Lower line-of-sight velocities towards the Wing and the larger\nvelocities just beyond the SW end of the SMC Bar are probably associated with\nstellar components of the Magellanic Bridge and Counter-Bridge, respectively.\nOur results reinforce the notion that the intermediate-age stellar population\nof the SMC is subject to substantial stripping by external forces.\n