DYNAMICAL MODELING OF THE MERGER OF THE SAGITTARIUS DWARF-MILKY WAY SYSTEM

Abstract

In hierarchical models of large-scale structure formation, most galaxies grow in clusters, where merging is an essential, if not dominant process. Understanding the merger process is greatly aided by examples in the vicinity of the Milky Way due to the availability of high quality positional and kinematic data. The Sagittarius Dwarf Galaxy at a distance of 25 kpc is a prime example. Detailed observations of stars in this dwarf galaxy and its associated tidal streams also provide a unique probe of the gravitational field of the Milky Way. To fully utilize this probe requires not only excellent observations but also accurate models of the merging event. We investigate the recent history of the Sagittarius Dwarf with the smoothed particle hydrodynamics/N-body GADGET-2 code, treating both the Milky Way and the dwarf galaxy as a particle system. In addition to considerations of particle number, gravitational softening, stability criteria such as Toomre Q parameter, and angular momentum transport, we add the distinction between static and dynamic potential differences as another aspect that deserves consideration. We find that the commonly employed approximation of a static Milky Way potential induces changes in the orbit corresponding to position changes on the sky of order 0.5°-1°. The weak dependence of the results on the live versus static Milky Way model justify the use of static potentials in situations which do not require very high positional accuracy.