Substructure and Tidal Streams in the Andromeda Galaxy and its Satellites

Abstract

Tidal streams from existing and destroyed satellite galaxies populate the outer regions of the Andromeda galaxy (M31). This inhomogeneous debris can be studied without of many of the obstacles that plague Milky Way research. We review the history of tidal stream research in M31, and in its main satellite galaxies. We highlight the numerous tidal streams observed around M31, some of which reside at projected distances of up to $\sim 120$ kpc from the center of this galaxy. Most notable is the Giant Stellar Stream, a signature of the most recent significant accretion event in the M31 system. This event involved an early-type progenitor of mass $\sim 10^{9}$ M$_{\odot }$ that came within a few kpc of M31’s center roughly a gigayear ago; almost all of the inner halo (R  ≤ 50 kpc) debris in M31 can be tied either directly or indirectly to this event. We draw attention to the fact that most of M31’s outer halo globular clusters lie preferentially on tidal streams and discuss the potential this offers to use these systems as probes of the accretion history. Tidal features observed around M33, M32, NGC 205 and NGC 147 are also reviewed. We conclude by discussing future prospects for this field.