Special TAPIR Seminar

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 predicted and observed properties of their satellite galaxy populations constitute some of the most important open challenges to galaxy formation models. Yet, to-date, our observational insight in both of these regimes has largely been limited to the Local Group. To address this deficit, we have surveyed the halos and satellite populations of 4 nearby galaxies with the Subaru Hyper Suprime-Cam. Using deep resolved stellar populations, we measure the stellar halo to unprecedented surface brightness depths and detect satellites down to ultra-faint dwarfs. Though the survey is ongoing, we have already gained a number of important insights. Surprisingly, 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 its current interaction with M82 will eventually result in one of the most massive stellar halos in the nearby universe, rivaling M31. Yet, it has experienced a surprisingly quiet accretion history to this point, indicating that this single event will completely transform its stellar halo. This is the first time such a large sample of systems, for which the stellar halos and satellites can be measured to a similar level as the Local Group, has been compiled. Combined with ever-improving models, these exquisite datasets will generate never-before-gained insight into the formation of systems like the MW. Abstract to be announced