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

TL;DR

The paper argues that by the 2040s–2050s, while global surveys will map the Milky Way extensively, robust constraints on the Galaxy’s detailed assembly history and dark matter granularity require high-resolution spectroscopy of faint main-sequence and turnoff stars in the northern sky, particularly toward the Galactic anticenter. It proposes a Northern Hemisphere 30 m-class telescope with wide-field, high-resolution, highly multiplexed spectroscopy to obtain $R \sim 30{,}000$ spectra for $ obreak 10^{5}$–$10^{6}$ faint stars out to $\sim$150–200 kpc, enabling chemodynamical tagging, halo substructure characterization, and outer-disk tomography. The paper outlines representative northern surveys (Anticenter Tomography, Northern Halo Deep Survey, Northern Streams/Substructure, Northern Cluster Chronology) and emphasizes synergy with the southern ELT, Gaia, Rubin, Euclid, and Roman to achieve genuinely all-sky Milky Way archaeology and tight constraints on dark matter granularity. The envisioned facility would thus play a pivotal role in mapping the Galactic potential, tracing accretion history, and providing high-value follow-up on rare tracers, maximizing the scientific return of the 2040s-era data ecosystem.

Abstract

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.