The Satellites and Stellar Halos of Nearby Milky Way-Mass Galaxies

Abstract

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, I have conducted a survey of the halos and satellite populations of two nearby galaxies with the Subaru Hyper Suprime-Cam. Using deep resolved stellar populations allows measurement of the stellar halo to unprecedented surface brightness depths and the detection satellites down to the threshold of ultra faint dwarf galaxies, such as the faint M81 satellite d1005+68. Though the survey is ongoing, these systems have yielded a number of important insights. 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, I have 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. Lastly, building on these revelations of the unexpected diversity in satellite populations and merger histories of MW-mass systems, and including the numerous other recent satellite and stellar halo surveys of nearby systems, I investigate a possible relationship between these two fundamental galactic components. Using data from seven nearby systems, I find a strong and previously-unknown positive correlation between their satellite populations and the mass of their most massive merger events. Surprisingly, current flagship galaxy formation simulations fail to reproduce this relationship — an acute shortcoming of the theoretical framework upon which our current galaxy formation paradigm is built.