Unraveling the Galactic Stellar Halo with the H3 Survey

The Milky Way has assimilated smaller systems throughout its history. Its outer reaches (the stellar halo) are a melting pot for stars that were born elsewhere, but now call the MW their home. Despite being scattered across the Galaxy, these migrant stars retain memory of their common origin that can be accessed via their shared chemistry and 3D positions + 3D velocities. In this talk I will present results from the H3 Survey—the first large-scale (125k+ stars observed) spectroscopic survey that uses Gaia data to preferentially target halo stars. I will present a comprehensive anatomy of our Galaxy out to 50 kpc, identifying its various constituents in full 6D phase-space & chemical space. I will highlight the following new structures in the distant Galaxy: (i) Aleph ([Fe/H] = -0.5), a low-eccentricity structure that rises a surprising 10 kpc off the plane, (ii) and (iii) Arjuna ([Fe/H] = -1.2) and I'itoi ([Fe/H] < -2), which comprise the high-energy retrograde halo, and (iv) Wukong ([Fe/H] = -1.6), a prograde phase-space overdensity. Stars born within the Galaxy are a major component at |Z|∼2 kpc (≈60%), but their relative fraction declines sharply to <5% past 15 kpc, pointing to a relatively quiescent recent accretion history. Beyond 15 kpc, >80% of the halo is built by two massive accreted dwarfs: GSE ([Fe/H]=−1.2) within 25 kpc, and Sgr ([Fe/H]=−1.0) beyond 25 kpc. This explains the relatively high overall metallicity of the halo ([Fe/H]≈−1.2). We attribute >95% of the sample to distinct structures, pointing to a halo built entirely from accreted dwarfs and heating of the disk. The Milky Way progenitors unearthed in this work provide a unique, ultra-resolved glimpse into the high-redshift universe that will be inaccessible even to JWST.