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.

Here we provide the most comprehensive determinations of the rest-frame UV luminosity function (LF) available to date with the Hubble Space Telescope (HST) at z ∼ 2–9. Essentially all of the noncluster extragalactic legacy fields are utilized, including the Hubble Ultra Deep Field, the Hubble Frontier Fields parallel fields, and all five CANDELS fields, for a total survey area of 1136 arcmin2. Our determinations include galaxies at z ∼ 2–3 leveraging the deep HDUV, UVUDF, and ERS WFC3/UVIS observations available over an ∼150 arcmin2 area in the GOODS-North and GOODS-South regions. All together, our collective samples include >24,000 sources, >2.3× larger than previous selections with HST. We identify 5766, 6332, 7240, 3449, 1066, 601, 246, and 33 sources at z ∼ 2, 3, 4, 5, 6, 7, 8, and 9, respectively. Combining our results with an earlier z ∼ 10 LF determination by Oesch et al., we quantify the evolution of the UV LF. Our results indicate that there is (1) a smooth flattening of the faint-end slope α from α ∼ −2.4 at z ∼ 10 to α ∼ −1.5 at z ∼ 2, (2) minimal evolution in the characteristic luminosity M* at z ≥ 2.5, and (3) a monotonic increase in the normalization log 10 ϕ * from z ∼ 10 to 2, which can be well described by a simple second-order polynomial, consistent with an “accelerated” evolution scenario. We find that each of these trends (from z ∼ 10 to 2.5 at least) can be readily explained on the basis of the evolution of the halo mass function and a simple constant star formation efficiency model.

The protagonists of the last great phase transition of the universe -cosmic reionization -remain elusive. Faint star-forming galaxies are leading candidates because they are found to be numerous and may have significant ionizing photon escape fractions (f esc ). Here we update this picture via an empirical model that successfully predicts latest observations (e.g., the rapid drop in star-formation density (ρ SFR ) at z > 8). We generate an ionizing spectrum for each galaxy in our model and constrain f esc by leveraging latest measurements of the reionization timeline (e.g., Lyα damping of quasars and galaxies at z > 7). Assuming a constant f esc across all sources at z > 6, we find M UV < −13.5 galaxies need f esc =0.21 +0.06 −0.04 to complete reionization. The inferred IGM neutral fraction is [0.9, 0.5, 0.1] at z = [8.2, 6.8, 6.2] ± 0.2, i.e., the bulk of reionization transpires rapidly in 300 Myrs, driven by the z > 8 ρ SFR and favored by high neutral fractions (∼60−90%) measured at z ∼ 7 − 8. Inspired by the emergent sample of Lyman Continuum (LyC) leakers spanning z ∼ 0−6.6 that overwhelmingly displays higher-than-average star-formation surface density (Σ SFR ), we propose a physically motivated model relating f esc to Σ SFR and find f esc ∝ Σ 0.4±0.1 SFR . Since Σ SFR falls by ∼ 2.5 dex between z = 8 and z = 0, our model explains the humble upper limits on f esc at lower redshifts and its required evolution to f esc ∼ 0.2 at z > 6. Within this model, strikingly, <5% of galaxies with M UV < −18 and log(M /M ) > 8 (the 'oligarchs') account for 80% of the reionization budget -a stark departure from the canonical 'democratic' reionization led by copious faint sources. In fact, faint sources (M UV >−16) must be relegated to a limited role in order to ensure high neutral fractions at z = 7 − 8. Shallow faint-end slopes of the UV luminosity function (α UV > −2) and/or f esc distributions skewed toward massive galaxies produce the required late and rapid reionization. We predict LyC leakers like COLA1 (z = 6.6, f esc ∼ 30%, M UV = −21.5) become increasingly common towards z ∼ 6 and that the drivers of reionization do not lie hidden across the faint-end of the luminosity function, but are already known to us.

The first few 100 Myr at z > 10 mark the last major uncharted epoch in the history of the universe, where only a single galaxy (GN-z11 at z ≈ 11) is currently spectroscopically confirmed. Here we present a search for luminous z > 10 galaxies with JWST/NIRCam photometry spanning ≈1–5 μm and covering 49 arcmin2 from the public JWST Early Release Science programs (CEERS and GLASS). Our most secure candidates are two M UV ≈ −21 systems: GLASS-z12 and GLASS-z10. These galaxies display abrupt ≳1.8 mag breaks in their spectral energy distributions (SEDs), consistent with complete absorption of flux bluewards of Lyα that is redshifted to z = 12.4 − 0.3 + 0.1 and z = 10.4 − 0.5 + 0.4 . Lower redshift interlopers such as quiescent galaxies with strong Balmer breaks would be comfortably detected at >5σ in multiple bands where instead we find no flux. From SED modeling we infer that these galaxies have already built up ∼109 solar masses in stars over the ≲300–400 Myr after the Big Bang. The brightness of these sources enable morphological constraints. Tantalizingly, GLASS-z10 shows a clearly extended exponential light profile, potentially consistent with a disk galaxy of r 50 ≈ 0.7 kpc. These sources, if confirmed, join GN-z11 in defying number density forecasts for luminous galaxies based on Schechter UV luminosity functions, which require a survey area >10× larger than we have studied here to find such luminous sources at such high redshifts. They extend evidence from lower redshifts for little or no evolution in the bright end of the UV luminosity function into the cosmic dawn epoch, with implications for just how early these galaxies began forming. This, in turn, suggests that future deep JWST observations may identify relatively bright galaxies to much earlier epochs than might have been anticipated.

Several lines of evidence suggest that the Milky Way underwent a major merger at z ∼ 2 with the Gaia-Sausage-Enceladus (GSE) galaxy. Here we use H3 Survey data to argue that GSE entered the Galaxy on a retrograde orbit based on a population of highly retrograde stars with chemistry similar to the largely radial GSE debris. We present the first tailored N-body simulations of the merger. From a grid of ≈500 simulations we find that a GSE with M ⋆ = 5 × 108 M ⊙, M DM = 2 × 1011 M ⊙ best matches the H3 data. This simulation shows that the retrograde stars are stripped from GSE’s outer disk early in the merger. Despite being selected purely on angular momenta and radial distributions, this simulation reproduces and explains the following phenomena: (i) the triaxial shape of the inner halo, whose major axis is at ≈35° to the plane and connects GSE’s apocenters; (ii) the Hercules-Aquila Cloud and the Virgo Overdensity, which arise due to apocenter pileup; and (iii) the 2 Gyr lag between the quenching of GSE and the truncation of the age distribution of the in situ halo, which tracks the lag between the first and final GSE pericenters. We make the following predictions: (i) the inner halo has a “double-break” density profile with breaks at both ≈15–18 kpc and 30 kpc, coincident with the GSE apocenters; and (ii) the outer halo has retrograde streams awaiting discovery at >30 kpc that contain ≈10% of GSE’s stars. The retrograde (radial) GSE debris originates from its outer (inner) disk—exploiting this trend, we reconstruct the stellar metallicity gradient of GSE (−0.04 ± 0.01 dex r 50 − 1 ). These simulations imply that GSE delivered ≈20% of the Milky Way’s present-day dark matter and ≈50% of its stellar halo.

The archeological record of stars in the Milky Way opens a uniquely detailed window into the early formation and assembly of galaxies. Here we use 11,000 main-sequence turn-off stars with well-measured ages, [ ] Fe H , [ ] a Fe , and orbits from the H3 Survey and Gaia to time the major events in the early Galaxy. Located beyond the Galactic plane,kpc 4, this sample contains three chemically distinct groups: a low-metallicity population, and low-α and high-α groups at higher metallicity. The age and orbit distributions of these populations show that (1) the high-α group, which includes both disk stars and the in situ halo, has a star formation history independent of eccentricity that abruptly truncated 8.3±0.1 Gyr ago (z;1); (2) the low-metallicity population, which we identify as the accreted stellar halo, is on eccentric orbits and its star formation truncated -+ 10.2. 0.1 0.2 Gyr ago (z;2); (3) the low-α population is primarily on low-eccentricity orbits and the bulk of its stars formed less than 8 Gyr ago. These results suggest a scenario in which the Milky Way accreted a satellite galaxy at z≈2 that merged with the early disk by z≈1. This merger truncated star formation in the early high-α disk and perturbed a fraction of that disk onto halo-like orbits. The merger enabled the formation of a chemically distinct, low-α disk at z1. The lack of any stars on halo-like orbits at younger ages indicates that this event was the last significant disturbance to the Milky Way disk.

Strong gravitational lensing provides a powerful probe of the physical properties of quasars and their host galaxies. A high fraction of the most luminous high-redshift quasars was predicted to be lensed due to magnification bias. However, no multiple imaged quasar was found at z > 5 in previous surveys. We report the discovery of J043947.08+163415.7, a strongly lensed quasar at z = 6.51, the first such object detected at the epoch of reionization, and the brightest quasar yet known at z > 5. High-resolution HST imaging reveals a multiple imaged system with a maximum image separation θ ∼ 0.2 , best explained by a model of three quasar images lensed by a low luminosity galaxy at z ∼ 0.7, with a magnification factor of ∼ 50. The existence of this source suggests that a significant population of strongly lensed, high redshift quasars could have been missed by previous surveys, as standard color selection techniques would fail when the quasar color is contaminated by the lensing galaxy.

Modern theories of galaxy formation predict that the Galactic stellar halo was hierarchically assembled from the accretion and disruption of smaller systems. This hierarchical assembly is expected to produce a high degree of structure in the combined phase and chemistry space; this structure should provide a relatively direct probe of the accretion history of our Galaxy. Revealing this structure requires precise 3D positions (including distances), 3D velocities, and chemistry for large samples of stars. The Gaia satellite is delivering proper motions and parallaxes for > 1 billion stars to G ≈ 20. However, radial velocities and metallicities will only be available to G ≈ 15, which is insufficient to probe the outer stellar halo ( 10 kpc). Moreover, parallaxes will not be precise enough to deliver high-quality distances for stars beyond ∼ 10 kpc. Identifying accreted systems throughout the stellar halo therefore requires a large ground-based spectroscopic survey to complement Gaia. Here we provide an overview of the H3 Stellar Spectroscopic Survey, which will deliver precise stellar parameters and spectrophotometric distances for ≈ 200, 000 stars to r = 18. Spectra are obtained with the Hectochelle instrument at the MMT, which is configured for the H3 Survey to deliver resolution R ≈ 23, 000 spectra covering the wavelength range 5150Å−5300Å. The survey is optimized for stellar halo science and therefore focuses on high Galactic latitude fields (|b| > 30 • ), sparsely sampling 15, 000 sq. degrees. Targets are selected on the basis of Gaia parallaxes, enabling very efficient selection of bone fide halo stars. The survey began in the Fall of 2017 and has collected 88,000 spectra to-date. All of the data, including the derived stellar parameters, will eventually be made publicly available via the survey website: h3survey.rc.fas.harvard.edu.