Conroy, CharlieHernquist, LarsHan, Jiwon2026-03-0420252025-10-032026Han, Jiwon. 2026. The Galactic Halo in Stars and Shadow. Doctoral Dissertation, Harvard University Graduate School of Arts and Sciences.32244617https://dash.harvard.edu/handle/1/42731101The 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.application/pdfenAstrophysicsThesis or Dissertation2026-03-040000-0002-6800-5778