Why the Northern Hemisphere Needs a 30-40 m Telescope and the Science at Stake: Galactic Archaeology from the Northern Sky

By the 2040s–50s, facilities such as \emph{Gaia}, WEAVE, 4MOST, Rubin, \emph{Euclid}, \emph{Roman}, and the ESO ELT will have transformed our global view of the Milky Way. Yet key questions will remain incompletely resolved: a detailed reconstruction of the Galaxy’s assembly from its earliest building blocks, and robust tests of dark matter granularity using the fine structure of the stellar halo and outer disk – particularly in the Galactic anticenter. Addressing these questions requires high-resolution spectroscopy of faint main-sequence stars (typically 1–2 mag below the turnoff) and turnoff stars ($r \sim 21$–23) in low-surface-brightness structures: halo streams and shells, ultra-faint dwarf galaxies, the warped and flared outer disk, and anticenter substructures. We argue that addressing this science case requires a 30,m-class telescope in the northern hemisphere, equipped with wide-field, highly multiplexed, high-resolution spectroscopic capabilities. Such a facility would enable (i) a Northern Halo Deep Survey of $\sim 10^{5}$–$10^{6}$ faint main-sequence and turnoff stars out to $\sim 150$–200,kpc, (ii) chemodynamical mapping of dozens of streams to measure perturbations from dark matter subhalos, and (iii) tomographic studies of the anticenter and outer disk to disentangle perturbed disk material from accreted debris. A northern 30,m telescope would provide the essential complement to ESO’s southern ELT, enabling genuinely all-sky Milky Way archaeology and delivering stringent constraints on the small-scale structure of dark matter.

💡 Research Summary

The white paper presents a compelling case that, even after the transformative data from Gaia, Rubin, Euclid, Roman, WEAVE, 4MOST, and the ESO Extremely Large Telescope (ELT) become available in the 2040s–2050s, critical gaps will remain in our understanding of the Milky Way’s detailed assembly history and the granularity of its dark‑matter halo. The authors argue that these gaps stem from the inability of existing 8–10 m facilities, and even the ELT’s targeted high‑resolution spectroscopy, to obtain high‑resolution (R ≈ 30 000), high‑signal‑to‑noise spectra for the faint main‑sequence and turn‑off stars (r ≈ 21–23) that dominate the low‑surface‑brightness components of the outer halo, stellar streams, ultra‑faint dwarf galaxies, and the warped, flared outer disk—particularly in the Galactic anticenter region that is best accessed from the northern hemisphere.

To fill this void, the paper proposes the construction of a 30–40 m class telescope in the northern hemisphere equipped with a wide‑field, highly multiplexed, high‑resolution spectroscopic instrument. The instrument would need to deliver a field of view of order one to two square degrees, multiplexing capabilities of several thousand fibers (or integral‑field units), and a spectral resolution of R ≈ 30 000 across the optical band (0.8–1.0 µm) with an optional near‑infrared extension to 1.2 µm. Such specifications would provide the photon‑collecting power and survey efficiency required to obtain ∼10⁵–10⁶ high‑quality spectra of faint halo tracers out to 150–200 kpc.

Three flagship survey programs are outlined: (i) a Northern Halo Deep Survey that maps the chemodynamical properties of a massive sample of faint main‑sequence and turn‑off stars, enabling a six‑dimensional reconstruction of the halo’s mass distribution, shape, and time‑dependent growth; (ii) a Northern Streams and Substructure Programme that chemically tags dozens of streams and shells, measuring perturbations from dark‑matter subhalos down to 10⁶–10⁸ M⊙ and providing a direct test of ΛCDM predictions on small scales; and (iii) an Anticenter and Outer‑Disk Tomography that disentangles warped‑disk material from accreted debris, characterizing the warp, flare, and low‑latitude overdensities in the outer disk.

The authors emphasize the complementarity with the southern ELT. While the ELT will excel at ultra‑deep, high‑resolution spectroscopy of selected fields, it cannot efficiently cover the wide northern sky regions that host many of the most informative structures. A northern 30 m facility would thus enable truly all‑sky Milky Way archaeology, allowing the combination of Gaia’s astrometry, Rubin’s deep photometry, and space‑based spectroscopy with ground‑based high‑resolution chemical tagging.

The paper also notes the recent stagnation of the Maunakea Spectroscopic Explorer (MSE) project, arguing that the timing is ripe for a new northern large‑aperture spectroscopic platform. By providing the missing high‑resolution, high‑multiplex capability, the proposed telescope would not only deliver the core science goals outlined above but also serve as a rapid follow‑up engine for rare, high‑value targets (e.g., extremely metal‑poor stars, distant blue horizontal‑branch tracers) identified by the aforementioned surveys.

In summary, the white paper convincingly demonstrates that a 30–40 m telescope in the northern hemisphere, equipped with a wide‑field, high‑resolution, highly multiplexed spectrograph, is essential to achieve a quantitative, time‑resolved, all‑sky picture of the Milky Way’s formation and to place stringent constraints on the small‑scale structure of dark matter. The proposed facility would fill a critical observational gap, synergize with the ELT and forthcoming space missions, and secure the scientific return of the next generation of Galactic surveys.