The Satellites and Stellar Halos of Nearby MW-mass Galaxies

The outskirts of galaxies like the Milky Way (MW) are important testing grounds for our understanding of galaxy formation and evolution. Models and observations agree that their vast accreted halos, while incredibly faint and difficult to observe, tantalizingly encode the properties of past merger events. Further, discrepancies between the predicted properties and distribution of their satellite galaxy populations constitute one of the most important open challenges to galaxy formation models. Yet, to-date, our observational insight in both of these regimes has been limited to the Local Group. To address this deficit, we have conducted a survey of the halos and satellite populations of 4 nearby galaxies with the Subaru Hyper Suprime-Cam. Using deep resolved stellar populations, we are able to measure the stellar halo to unprecedented surface brightness depths and detect satellites down to the ultra faint dwarf regime. Though the survey is ongoing, we have gained a number of important insights into these systems. For example, the sparse satellite population of the ‘lonely giant’ M94 challenges all current model predictions, and suggests that low-mass galaxy formation could be more stochastic than previously thought. Additionally, we have recently used the stellar halo of M81 to show that it has experienced a surprisingly quiet accretion history to this point. Yet, its current interaction with M82 (and NGC 3077) will eventually result in one of the most massive stellar halos in the nearby universe, rivaling the behemoth M31. This is the first time such a large sample of systems, for which the stellar halos and satellites can be measured to a similar quality as the Local Group, has been compiled. Armed with this first-of-its-kind sample, powerful and unexpected relationships emerge between satellite populations and merger history. These results present even more acute challenges to current models, and suggest a fundamental gap in our understanding of the formation of MW-mass systems.