Computer models of the Sagittarius dwarf interaction with the Milky Way

The interaction between the dwarf galaxy in Sagittarius and the Milky Way Galaxy has been modelled with a parallel computer implementation of an N-body treecode. Models are made that reproduce the observed position, size, velocity, proper motion and velocity gradient of the dwarf in its likely pre-disc encounter, and other models are studied in which the dwarf has just passed through the disc. Several observable differences between these cases are found. In the pre-collision case, the dwarf is bound to the Milky Way and it passed through the disc previously 1.7 x 10(8) yr ago in the anticentre direction. It will cross through the disc again in 3.5 x 10(7) yr. Each disc crossing spins up the dwarf core and tidally stretches its outer envelope, leaving the core slightly less bound. If such disc crossings are common for dwarfs near large galaxies, then the likely removal of gas from the dwarf at each crossing, combined with the observed high dark matter fraction for most dwarfs, implies that dark matter is not in the form of undetected cold gas. In the preferred model, the outer envelope of the dwarf includes the positions and velocities of the globular clusters NGC 6715, Terzan 7, Terzan 8 and Arp 2, which implies that these clusters could be fragments of a previously larger system. They could also be old detached star formation cores that formed in the Sagittarius dwarf during previous disc crossings.