A DYNAMICAL STUDY OF THE SEXTANS DWARF SPHEROIDAL GALAXY

We have observed 26 giant stars in the Sextans dwarf spheroidal (dSph) galaxy and obtained high-quality spectra for 21 of these. We have multi-epoch spectra for nine stars from which two possible binary stars have been found. The velocity dispersion of the sample calculated by a maximum likelihood method is 7.0 +/- 1.3/1.0 km s-1 and the mean velocity is 224.4 +/- 1.6 km s-1. There is no sign of rotation about any axis within the errors of the measurements, the formally derived value around the minor axis being 0.4 km s-1 at 300 pc from the axis. It is possible that binary stars may be making a significant contribution to the velocity distribution or that Sextans may be being tidally disrupted. If further work eliminates both these possibilities then, assuming isotropy in the velocity dispersion and dynamical equilibrium, the core mass-to-light ratio is 124(+85/-60) M./L. and the total mass-to-light ratio is 121(+84/-58) M./L., implying the presence of large quantities of dark matter.