Stellar Tidal Streams in External Galaxies
  To place the highly substructured stellar halos of the Milky Way and M31 in a
larger context of hierarchical galaxy formation, it is necessary to understand
the prevalence and properties of tidal substructure around external galaxies.
This chapter details the current state of our observational knowledge of
streams in galaxies in and beyond the Local Group, which are studied both in
resolved stellar populations and in integrated light. Modeling of individual
streams in extragalactic systems is hampered by our inability to obtain
resolved stellar kinematics in the streams, though many streams contain
alternate luminous kinematic tracers, such as globular clusters or planetary
nebulae. We compare the observed structures to the predictions of models of
galactic halo formation, which provide insight in the number and properties of
streams expected around Milky Way like galaxies. More specifically, we discuss
the inferences that can be made about stream progenitors based only on observed
morphologies. We expand our discussion to consider hierarchical accretion at
lower mass scales, in particular the observational evidence that substructure
exists on smaller mass scales and the effects accretion events may have on the
evolution of dwarf galaxies (satellite or isolated). Lastly, we discuss
potential correlations between the presence of substructure in the halo and the
structural properties of the disk. While many exciting discoveries have been
made of tidal substructures around external galaxies, the "global" questions of
galaxy formation and evolution via hierarchical accretion await a more complete
census of the low surface brightness outskirts of galaxies in and beyond the
Local Group.
