The structure and dynamics of the Small Magellanic Cloud (SMC) remain poorly understood, despite the increased volume and quality of data that have become available in recent years. A fundamental question is the extent to which the dynamics of the SMC are influenced by its interaction with the Large Magellanic Cloud (LMC). Most of the kinematical studies to date concentrated on young stars and gas (see e.g. Torres & Carranza 1987 for a review; Mathewson et al. 1988), or they were limited to the central regions of the SMC. However, most of the gravitating mass of the SMC is associated with the intermediate-age and old populations. Therefore a complete understanding of the SMC dynamics and of the interaction between the LMC and the SMC requires kinematical studies of relatively old populations. Another reason for selecting old stars for such a study is that the kinematics of gas and young stars (formed from it) can be significantly affected by hydrodynamic processes. These may be associated both with interactions between the gaseous components of the two galaxies during a close encounter, and with the star formation mechanisms themselves, through the interactions of stellar winds and supernovae with the interstellar medium. The very complicated picture of the kinematical field in the SMC, arising from the latter processes, can be seen clearly in the new high resolution 21-cm hydrogen maps constructed by Staveley-Smith et al. (1995).
There have been four major studies of the kinematics of older populations in the SMC published to date. Hardy et al. (1989) derived radial velocities for a sample of 150 carbon stars located in the central 1-2 kpc of the SMC. They found no evidence for rotation or of velocity splitting in their sample. Similar results had been found earlier by Dopita et al. (1985) from a study of the kinematics of 44 planetary nebulae lying mostly in the main body of the SMC. It is difficult however to extrapolate these results to the SMC as a whole, the radial extent of which is >6 kpc in projection (see e.g. Morgan & Hatzidimitriou 1995). In the outer regions of the SMC there have been two kinematical studies of old stars. However, both were restricted to small regions, and contain relatively small numbers of stars. Suntzeff et al. (1986) derived the radial velocities of 11 "halo'' giants located close to the star cluster NGC 121. There was no evidence in their sample of a bimodal velocity distribution, though the sample was too small for any firm results. Hatzidimitriou et al. (1993) studied the radial velocity distribution of a sample of 29 red horizontal-branch stars to the NE of the SMC, distributed within a 40 arcmin field, at a projected distance of kpc from the optical centre of the SMC. The sample was located in a region of alleged large depth along the line of sight (Hatzidimitriou & Hawkins 1989). The data showed a well-defined correlation between distance along the line of sight and radial velocity, which was tentatively interpreted as the result of the tidal interaction between the LMC and the SMC. This result is suggestive of disturbed dynamics along the line of sight, at least in one particular direction. It reinforces the usefulness of kinematical studies of older stars in the outer parts of the SMC, where the stars are less strongly bound to their parent galaxy and hence may be more easily seen to reflect the gravitational effects of the SMC-LMC interaction. Nevertheless, it is obvious that kinematical information for much larger samples of old stars distributed over larger areas is necessary to understand properly the kinematics of the SMC.
Carbon stars are very well suited for this type of study: by being easily found and unambiguously recognised they provide an observationally well-defined sample; astrophysically they belong to a reasonably well-understood intermediate-age population and are often used as tracers of such populations in nearby galaxies. In the Magellanic Clouds in particular, carbon stars are sufficiently numerous and bright to be particularly useful for kinematical and dynamical studies. It is important to note that the population of carbon stars in the SMC maps closely the distribution of intermediate-age and old stars, as can be seen by a comparison of their spatial distribution with that of clump/red horizontal-branch stars, which are generally older than Gyr (Gardiner & Hatzidimitriou 1992; Morgan & Hatzidimitriou 1995) and with that of RR Lyrae stars (Hatzidimitriou 1993).
In this paper, we present the kinematical study of a sample of 71 outlying carbon stars in the SMC. We also observed a small sample of 16 carbon star candidates in the LMC halo on the side nearest to the SMC. Sect. 2 describes the observations and reductions; in Sect. 3 the resulting radial velocities and their accuracy are presented, while in Sect. 4 we discuss the significance of the results.