Galactic indigestion: Numerical simulations of the Milky Way's closest neighbor

Are dwarf spheroidal galaxies dark matter dominated? We present N-body simulations of the interaction between the Milky Way and its closest companion, the Sagittarius dwarf spheroidal galaxy, constrained by new kinematic, distance, and surface density observations (detailed in a companion paper). It is shown that there is no possible self-consistent solution to the present existence of the Sagittarius dwarf if its distribution of luminous matter traces the underlying distribution of mass. The luminous component of the dwarf galaxy must therefore be shielded within a small dark matter halo. Although at present we are unable to construct a fully self-consistent model that includes both the stellar and dark matter components, it is shown numerically that it is possible that a pure dark matter model, approximating the dark matter halo deduced for the Sagittarius dwarf from analytical arguments, may indeed survive the Galactic tides. The orbit of the Sagittarius dwarf around the Milky Way is considered, taking into account the perturbative effects of the Magellanic Clouds. It is shown that at the present time, the orbital period must be short, similar to 0.7 Gyr; the initial orbital period for a 10(9) M-. model will have been similar to 1 Gyr. It is found that a close encounter with the Magellanic Clouds may have occurred, although the chances of such an interaction affecting the orbit of the Sagittarius dwarf are negligible.