The debris structures described in this volume that have been discovered around the Milky Way and other galaxies are thought to be formed from the disruption of satellite stellar systems — dwarf galaxies or globular clusters — by galactic tidal fields. The total stellar mass in these structures is typically tiny compared to the galaxy they might be found around and it is easy to dismiss them as visually striking, but inconsequential. However, they are remarkably useful as probes of a galaxy's history (covered in this chapter) and mass distribution (covered in the next chapter). This power is actually a consequence of their apparent insignificance: their low contribution to the overall mass makes the physics that describes them both elegant and simple and this means that their observed properties are relatively easy to understand and interpret.
Figure 1 contrasts internal and external views of two examples of debris structures seen in numerical simulations: streamsstreams from the disruption of a satellite along a mildly eccentric orbit (lower panels) and shellsshells from the disruption of the same satellite along a much more eccentric orbit (upper panels; see Section 2 for a more detailed description of these simulations). This chapter first outlines our understanding of the formation of such debris structures (Section 3) and then goes on to explore what we can learn about dying and long-dead satellites from observations of their debris (Section 4) and the implications of the cosmological context for properties of the combined system of all debris structures that form our stellar halo (Section 5).
Figure 1. External (left panels) and internal (i.e. all-sky projections – right panels) views of two examples of debris structures seen in numerical simulations described in Section 2. Both debris structures were formed along orbits with the same energy as a circular orbit at radius 25 kpc in the parent potential. The top panels are for a 6.5 × 108 M⊙ satellite falling in along a highly eccentric orbit (angular momentum L / Lcirc = 0.1) and the bottom panels are for the same satellite on a more mildly eccentric orbit (L / Lcirc = 0.9).
The following chapter discusses what debris structures can tell us about the tidal field — i.e. the mass distribution of the galaxy that destroyed them.