Relating the Diverse Merger Histories and Satellite Populations of Nearby Galaxies

The hierarchical formation of galaxies like the Milky Way (MW) is a central prediction of the Lambda-Cold Dark Matter (LCDM) model. They are predicted and observed to host vast halos of stars accreted from disrupted dwarf galaxies, as well as rich dwarf satellite populations — both of which tantalizingly encode details of their formation histories. This regime has long been problematic for galaxy formation models, due to the required resolution, and observational progress has been largely constrained to the Local Group, due to the intrinsic faintness and large scales of both features. To address this observational deficit and generate more useful comparison samples for models, I have contributed to a field-wide effort to survey the halos and satellites of nearby MW-mass systems. With observations from the Subaru Hyper Suprime-Cam, we are able to measure nearby galaxies' stellar halos to unprecedented depths and detect their complete satellite populations. Combined with insight from similar studies in the community, we have learned that (1) the stellar halos of MW-analogs are remarkably diverse, and (2) their satellite galaxy populations are equally diverse. The diversity in the halo properties of these galaxies appears to correspond directly to a diversity in their largest mergers. Leveraging this dual insight, we find that the diversity in these two components are connected: the number of dwarf satellites a MW-like galaxy hosts is tightly correlated with the mass of its largest merger. I will show empirically that indeed the MW and the nearby galaxy M81 evolve along this relation throughout their largest mergers. Unexpectedly, this relationship is not predicted by high-resolution galaxy formation models. The difference in the observed and predicted connection between two fundamental outcomes of hierarchical assembly represents an important gap in our understanding of galaxy formation in LCDM.