A DYNAMICAL STUDY OF THE URSA-MINOR DWARF SPHEROIDAL GALAXY

We have observed 46 giant stars in the Ursa Minor dwarf spheroidal (dSph) galaxy and obtained high-quality spectra for 35 of these. The velocity dispersion of the sample calculated by a maximum likelihood method is 7.5(-0.9)(+1.0) km s(-1) and the mean velocity is -249.2 +/- 1.5 km s(-1). There is evidence for rotation around the major axis: a straight line fit, equivalent to solid body rotation, gives a gradient of -4.7(-2.2)(+2.0) km s-1 per 100 pc from the axis, where the positive distances are to the north-west of the major axis. This discovery adds evidence to the suggestion that Ursa Minor may be being tidally disrupted. If further work eliminates this possibility, then, assuming dynamical equilibrium, isotropy in the velocity dispersion (which is 6.7(-0.8)(+0.9) km s(-1) when the rotation has been subtracted) and a negligible contribution from binary stars, the core mass-to-light ratio is 59(-25)(+41) M./L. and the total mass-to-light ratio is 40(-17)(+29)M./L., implying the present of large quantities of dark matter.