Low surface brightness features, including “tails” and “bridges,” are visible in highly disturbed galaxies that result from a major merger or strong encounter (Arp 1966, Toomre & Toomre 1972). Similarly, diffuse stellar streams and shells around massive elliptical galaxies have been known for decades, and are attributed either to the accretion of smaller disk galaxies (Quinn 1984) or to recent, “in-situ” star formation from gas that was already contained within the galaxy (Fabian et al. 1980). Schweizer & Seitzer (1988) extended these observations to early spiral galaxies, and suggested that the “ripples,” as they called the shell-like features, were formed through mass transfer from nearby galaxies, in addition to wholesale mergers.
It has only been in recent years, with the advent of wide-area, deep photometric surveys, that the number and variety of stellar substructures (resulting from the tidal disruption of dwarf galaxies and globular clusters in “minor mergers”) threading the halos of the Milky Way and Andromeda (M31) galaxies has become apparent. These stellar substructures can be studied in detail and together describe the hierarchical merging history of the two dominant galaxies in the Local Group. However, placing them in the broader context of cosmological galaxy formation models requires a more general picture of halo substructure only feasible via the exploration of a large number of more distant systems. Only with such a dataset in hand is it possible to determine whether the Milky Way and M31 have experienced ‘typical' or ‘atypical' merging histories (e.g., Mutch et al. 2011).
Models (e.g., Johnston et al. 2008, Cooper et al. 2010) predict that a survey reaching a surface brightness of ∼ 29 mag arcsec−2 around ∼ 100 galaxies outside the Local Group should reveal many tidal features, perhaps as much as one detectable stream per galaxy. However, a suitably deep data set that is sensitive to low surface brightness features in a large number of galaxies does not yet exist, leaving the observational evidence needed to test these predictions incomplete. In the sections that follow, we will discuss the isolated discoveries of tidal debris structures in external galaxies and their overall utility for elucidating general stellar tidal features.