Unlocking the physics of dwarf galaxies in the 2040s: The case for a next-generation wide-field spectroscopic facility with fibres and IFUs

Dwarf galaxies ($M_{\star} \lesssim 10^{9} M_{\odot}$) are the most numerous galaxies in the Universe and critical probes of dark matter, baryonic feedback, and galaxy formation. Despite significant progress from wide-field imaging surveys, the majority of dwarf candidates beyond the Local Group will lack spectroscopic follow-up, leaving fundamental questions about their internal kinematics, stellar populations, chemical enrichment, and dark matter content unresolved. Existing and planned facilities cannot efficiently provide the necessary spectroscopy for low-surface-brightness dwarfs over wide areas. We advocate for a dedicated large-aperture ($\geq 20$ m), wide-field, highly multiplexed spectroscopic facility with deployable or monolithic IFUs, capable of high signal-to-noise observations down to $I_{\rm E} \gtrsim 22-23$ mag. Such a facility would enable transformative studies of dark matter cores, baryonic feedback, tidal interactions, environmental effects, and stellar populations, extending the spectroscopic exploration of low-mass galaxies to $z \sim 1.5$, and providing decisive tests of $Λ$CDM and alternative dark matter models. Beyond dwarfs, this capability would impact galaxy evolution, strong and weak lensing studies, and cosmology, ensuring that imaging data from the 2030s and 2040s can be fully exploited.

💡 Research Summary

This white paper makes a compelling case that a dedicated, next‑generation spectroscopic facility is essential for unlocking the physics of dwarf galaxies in the 2040s. Dwarf galaxies (stellar mass ≲ 10⁹ M⊙) are the most numerous galaxies and uniquely sensitive to the microphysics of dark matter, baryonic feedback, tidal interactions, and reionization. While the ΛCDM paradigm predicts a rich population of low‑mass subhalos, long‑standing small‑scale tensions—such as the missing‑satellites problem, the too‑big‑to‑fail issue, the cusp‑core discrepancy, and the existence of planar satellite structures—remain unresolved because current spectroscopic samples are limited to the Local Group. The authors argue that extending dwarf‑galaxy studies to the broader Local Volume (z < 0.2) and even to z ≈ 1.5 is crucial for testing whether these tensions are universal or a peculiarity of the Milky Way environment.

The paper outlines the dramatic increase in dwarf‑galaxy candidates expected from upcoming imaging surveys. Euclid’s 14 000 deg² Wide Survey, combined with LSST, will deliver more than two million dwarf candidates, with surface‑brightness limits reaching μr ≈ 29–30 mag arcsec⁻². Existing wide‑field spectroscopic facilities (4MOST, DESI, WEAVE, PFS) lack either the depth (I ≈ 22–23 mag) or the multiplexing required to obtain high‑S/N spectra for such faint, low‑surface‑brightness objects. An 8‑m class telescope reduces exposure times but still demands prohibitive total integration time for a million‑object sample. The ELT, despite its enormous light‑gathering power, cannot provide the necessary multiplexing to build statistically significant samples over thousands of square degrees.

Consequently, the authors propose a new telescope with a primary mirror of at least 20 m, a wide field of view (≈ 1–3 deg²), and a highly multiplexed fiber system capable of deploying thousands to tens of thousands of fibers simultaneously. The spectrograph should cover 3700–10 000 Å, offering low‑resolution (R ≈ 3000–4000) for bright dwarfs and medium‑resolution (R ≈ 5000–8000) for the faintest targets, enabling accurate redshifts, velocity dispersions down to σ ≈ 10 km s⁻¹, and detailed stellar‑population diagnostics (ages, metallicities, α‑enhancements). In addition, deployable or monolithic integral‑field units (IFUs) with sub‑arcsecond sampling are required to map internal kinematics, metallicity gradients, and tidal features in selected dwarfs.

The scientific payoff is multifold. High‑quality spectra will allow direct measurement of dark‑matter density profiles (cusp vs. core) across a wide range of environments, providing decisive tests of cold, warm, self‑interacting, or ultra‑light axion dark‑matter models. By resolving stellar velocity dispersions and rotation curves, the facility will address the too‑big‑to‑fail problem and quantify the impact of supernova and AGN feedback on dwarf potentials. Spatially resolved IFU data will reveal tidal stripping, kinematically decoupled cores, and the formation of ultra‑diffuse galaxies. Spectroscopy of associated globular clusters will enable independent dynamical mass estimates and chemical‑evolution studies, linking the formation histories of dwarfs and their cluster systems. Moreover, the massive spectroscopic dataset will improve photometric‑redshift calibrations for LSST and Euclid at the faint end, refine galaxy‑halo connection models, and enhance weak‑lensing and large‑scale‑structure cosmology.

Beyond dwarf galaxies, the proposed facility will serve a broad community: it will enable precise scaling‑relation studies from dwarf to massive galaxies, support strong‑lens modeling, map the cosmic web’s influence on galaxy evolution, and provide essential data for next‑generation cosmological analyses. In summary, without a ≥20 m, wide‑field, high‑multiplex spectroscopic telescope equipped with IFUs, the flood of imaging data anticipated for the 2030s and 2040s will remain largely uninterpreted, leaving the fundamental physics of dwarf galaxies—and their role as probes of dark matter and galaxy formation—unresolved well into the next decade.