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 atWe 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 | | Z 2 kpc (≈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 å ∼10 8 -10 9 M e ) 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.

The Galactic Archaeology with HERMES (GALAH) survey is a large-scale stellar spectroscopic survey of the Milky Way and designed to deliver chemical information complementary to a large number of stars covered by the Gaia mission. We present the GALAH second public data release (GALAH DR2) containing 342,682 stars. For these stars, the GALAH collaboration provides stellar parameters and abundances for up to 23 elements to the community. Here we present the target selection, observation, data reduction and detailed explanation of how the spectra were analysed to estimate stellar parameters and element abundances. For the stellar analysis, we have used a multi-step approach. We use the physics-driven spectrum synthesis of Spectroscopy Made Easy (SME) to derive stellar labels (T eff , log g, [Fe/H], [X/Fe], v mic , v sin i, A K S ) for a representative training set of stars. This information is then propagated to the whole survey with the data-driven method of The Cannon. Special care has been exercised in the spectral synthesis to only consider spectral lines that have reliable atomic input data and are little affected by blending lines. Departures from local thermodynamic equilibrium (LTE) are considered for several key elements, including Li, O, Na, Mg, Al, Si, and Fe, using 1D stellar atmosphere models. Validation tests including repeat observations, Gaia benchmark stars, open and globular clusters, and K2 asteroseismic targets lend confidence to our methods and results. Combining the GALAH DR2 catalogue with the kinematic information from Gaia will enable a wide range of Galactic Archaeology studies, with unprecedented detail, dimensionality, and scope.