Overall, the discovery (over the last two decades) of debris structures encircling our Galaxy has motivated the development of a fairly sophisticated understanding of and tools for modeling debris structures. Putting these models and observations in a cosmological context has led to a dramatic, local confirmation of the hierarchical contribution to structure formation on small-scales: there is broad agreement between the models and data with the picture that a large fraction of our stellar halo results from the accretion of smaller systems. However, the wide variety of possible accretion histories means that this demonstration itself, while interesting, does not place strong constraints on cosmological models.
Two aspects of studies of debris around the Milky Way remain ripe for further exploration with near-future data. First, we can use stars from disrupted satellites to recover the accretion history of our Galaxy. The unbound structures apparent in current data sets represent the most dominant events accreted in the last several billion years. Data sets that will be available in the near future will enable the identification of debris that is either older (i.e. more fully phase-mixed) and from lower luminosity objects, both because of the large numbers of stars that will be catalogued (e.g. by Gaia) and because of the additional data dimensions that will have accurate measurements (e.g. proper motions from Gaia and detailed chemical abundances from the GALAH survey). Most importantly, the additional data dimensions will allow the calculation of quantities that are likely to be conserved during the lifetime of the stars (e.g. orbital properties such as energy and actions as well as chemical composition), which will further enable the “tagging” of these stars into the groups in which they originally formed. We can look forward to using these approaches to look much further back to the very earliest epochs of galaxy formation using stellar surveys.
While the accretion history of our Galaxy will play a key part in understanding the specific story of its formation and evolution, it is unlikely to have broader implications for the histories of other Milky-Way-sized galaxies, which are expected to have accretion histories that vary widely. However, a by-product of the accretion history will be the luminosity function of smaller infalling galaxies, along with the identification of stellar populations associated with these different mass objects looking back to the higher redshifts at which they were accreted. Such small galaxies will be very hard (if not impossible) to observe at higher redshift even with the next generation of telescopes. Hence the second aspect of debris studies that should be further explored is what we can learn about the evolution of dwarf galaxies over cosmic time from our stellar halo. In particular, what can the stellar populations tell us about the baryonic physics of galaxy formation within small dark matter halos — the processes of gas inflow, star formation, and feedback that remain key challenges in understanding galaxies in the Universe today?
KVJ gratefully acknowledges David Hendel's work in generating the simulations and related figures used in section 3 of this chapter. KVJ thanks her postdocs and graduate students for invaluable discussions through out the year (Andrea Kuepper, Allyson Sheffield, Lauren Corlies, Adrian Price-Whelan, David Hendel and Sarah Pearson). Her work on this volume was supported in part by NSF grant AST-1312196.