Evidence from the H3 Survey That the Stellar Halo Is Entirely Comprised of Substructure

Abstract

In the ΛCDM paradigm, the Galactic stellar halo is predicted to harbor the accreted debris of smaller systems. To identify these systems, the H3 Spectroscopic Survey, combined with Gaia, is gathering 6D phase-space and chemical information in the distant Galaxy. Here we present a comprehensive inventory of structure within 50 kpc from the Galactic center using a sample of 5684 giants at and . We identify known structures including the high-α disk, the in situ halo (disk stars heated to eccentric orbits), Sagittarius (Sgr), Gaia–Sausage–Enceladus (GSE), the Helmi Streams, Sequoia, and Thamnos. Additionally, we identify the following new structures: (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 along with Sequoia, and (iv) Wukong ([Fe/H] = −1.6), a prograde phase-space overdensity chemically distinct from GSE. For each structure, we provide [Fe/H], [α/Fe], and orbital parameters. Stars born within the Galaxy are a major component at (≈60%), but their relative fraction declines sharply to ≲5% past 15 kpc. Beyond 15 kpc, >80% of the halo is built by two massive (M ⋆ ∼ 10 8–10 9 M ⊙) 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 one of the listed structures, pointing to a halo built entirely from accreted dwarfs and heating of the disk.