The Galactic Halo in Stars and Shadow

Abstract

The Galactic halo encodes both the ancient assembly history of the Milky Way and its ongoing interactions with satellites and dark matter. In this dissertation, I present six studies that together develop a new picture of the halo as a tilted, broken, and dynamically complex structure. Using halo stars from the H3 Survey, I show that the inner halo—dominated by the Gaia–Sausage–Enceladus merger—is best described as a tilted triaxial ellipsoid with a doubly broken density profile. I further demonstrate that the halo exhibits strong kinematic asymmetries and that stellar orbits preserve signatures of a misaligned Galactic potential, providing new constraints on the shape and orientation of the surrounding dark matter halo. Through both controlled experiments and cosmological simulations, I establish that tilted halos are long-lived, common in a cosmological context, and naturally excite the Milky Way’s warp and flare, offering a resolution to the decades-old mystery of the Galactic warp. Finally, I explore hypervelocity stars as dynamical probes of the halo’s hidden components. By reconstructing their trajectories, I find that a subset originates not in the Galactic Center but in the Large Magellanic Cloud, providing strong evidence for a massive black hole in our nearest satellite and a new constraint on its past orbit. Together, these results reveal the Milky Way halo as a dynamic structure that records ancient mergers and present-day interactions—traced both in stars and in shadow.