While the study of tidal streams from major mergers/encounters (1:3 mass ratio) or even minor encounters (1:3 to 1:10 mass ratio) is an old field, the extension of these studies to those streams formed by satellite galaxies, a.k.a., micro-mergers (mass ratio < 1:10), is a relatively new area of exploration. Study of mergers on this mass scale provide a direct way of addressing some open questions on galactic formation and evolution. In the last decade, the observational effort has yielded an unprecedented sample of bright stellar streams in nearby spiral galaxies, including the discovery of observational analogs to the canonical morphologies found in N-body models of stellar halos (Johnston et al. 2008; see chapter 6). This offers a unique opportunity to study in detail the apparently still dramatic last stages of galaxy assembly in the local universe and to probe the anticipated estimates of frequency of tidal stellar features from the Λ-CDM paradigm for MW-sized galaxies. Moreover, these discoveries demonstrate the need for deep, wide-field imaging that pushes fainter than current surveys in order to visualize external galaxy halos on par with the highly substructured portrait of our own Milky Way and M31. Such studies will address the following key questions (among others) on several aspects of hierarchical galaxy formation:
The study of external tidal streams also has the potential to tackle a significant number of other topics that are the focus of current astrophysical research (e.g., stellar populations of halos, the resilience of the disks involved with minor mergers, accretion of globular clusters, induced star formation in streams, near-field cosmology, satellite dynamics, dark matter halo shapes, etc.). In particular, the interpretation of global properties of galaxy halos and outer disks from resolved stellar populations (from, e.g., the Hubble Space Telescope survey GHOSTS; Radburn-Smith et al. 2011, or the CALIFA high-resolution spectroscopy survey of nearby stellar systems; Sánchez et al. 2013) requires understanding the role and prevalence of tidal debris in galaxy halos. In addition, studying stellar population gradients along tidal streams via deep HST photometric data (see, e.g., Aloisi et al. 2005) will render important constraints on the effect of tides on the stellar formation history of dwarf galaxies. The panoramic view of tidal streams in external galaxies also offers an excellent opportunity to demonstrate tidal stripping of globular clusters formed in satellite galaxies, which may correspond to an important fraction of the globular cluster population of the host, as earlier proposed by Searle & Zinn (1978). Ultimately, the ideal scenario would require resolving stellar populations in large numbers of galaxies at distances of 10-20 Mpc, which will be feasible in the next one or two decades with thirty-meter class ground-based telescopes or the proposed suite of space-based instrumentation.
Finally, the future census of tidal streams and their properties will also provide an essential framework for exploring whether the Milky Way is a template for the archetypal spiral galaxy. The next generation of galactic surveys (LSST) and future astrometric space missions (Gaia) will dissect the structure and formation of the Milky Way with unprecedented detail, leading to a revolutionary improvement of our understanding of the Galaxy. In this regard, the study of these structures in external systems will be complementary in interpreting this local Galactic archaeological data in the context of galaxy formation and evolution, providing unique data in order to quantify how typical the Milky Way is with respect to other nearby galaxies of its type.
Acknowledgements JLC gratefully acknowledges support from the NSF under grants AST 09-37523 and AST 14-09421.