ON THE DYNAMICS OF THE SAGITTARIUS DWARF GALAXY

We use numerical simulations to test the feasibility of the suggestion by Ibata, Gilmore & Irwin that the excess population of stars which they discovered in the Sagittarius region may be the disrupted remains of a dwarf spheroidal galaxy. We find that a Fornax-like model for the pre-disruption system can indeed reproduce the data. However, the galaxy must be on a relatively short-period orbit with a pericentre of about 10 kpc and an apocentre of about 52 kpc, giving a current transverse velocity of 255 km s(-1) and a period of similar to 760 Myr. Furthermore, disruption must have occurred predominantly on the last pericentric passage rather than on the present one. The data are consistent with transverse motion either towards or away from the Galactic plane. These results depend primarily on the rotation curve of the Galaxy and are insensitive to the mass distribution in its outer halo or to the mass of its disc.