Spin-filament alignments to unravel galaxy evolution and model intrinsic alignments

By the 2040s, several all-sky surveys will have transformed our view of the large-scale structure. However, one of the major outstanding questions in astrophysics will remain: understanding how galaxies acquire and evolve their angular momentum and how this connects to the cosmic web. Measuring the alignments between galaxy spins and cosmic filaments across cosmic time, and understanding what this reveals about galaxy evolution, requires surveys that also characterise intrinsic alignments, i.e. correlations in galaxy shapes produced by the cosmic web itself rather than by lensing. Intrinsic alignments are a major source of systematic error in weak-lensing measurements of the fundamental parameters of the Universe. Addressing both questions together will necessitate new types of MOS surveys that combine kinematic information with high-completeness redshifts down to at least 24-25mag. To achieve our science goals, we require a new generation of wide-field spectroscopic facilities that can obtain spin-filament alignment measurements for millions of galaxies while simultaneously delivering sub-Mpc resolution of the cosmic web and spatially-resolved kinematics required to map the spin-filament connection at the level of individual galaxies within their local cosmic environment. Such a program would provide a unique legacy survey of galaxies and cosmic structures from kiloparsec to megaparsec scales, establishing ESO’s leadership in bridging the physics of galaxy evolution with the systematic-control requirements for Stage-IV cosmological surveys.

💡 Research Summary

**
The white paper presents a compelling case for a next‑generation spectroscopic facility that simultaneously delivers massive multiplexed redshift surveys and spatially‑resolved kinematics in order to measure galaxy spin–filament alignments across cosmic time and to construct physically motivated intrinsic‑alignment (IA) models. In the ΛCDM framework, dark‑matter halos acquire angular momentum through tidal torques, leading to a preferential alignment between a galaxy’s spin vector and the orientation of the filamentary structure in which it resides. Simulations and existing observations have shown a clear mass dependence: low‑mass, star‑forming galaxies tend to align parallel to filaments, while high‑mass, bulge‑dominated systems preferentially align perpendicularly. This “flip mass” is predicted to evolve with redshift, reflecting the changing balance between smooth gas accretion, merger‑driven torques, and feedback processes. Measuring this evolution up to at least z ≈ 1 would provide a direct observational probe of how galaxies acquire, lose, and re‑orient their angular momentum, thereby testing competing galaxy‑formation models.

Beyond galaxy evolution, spin–filament alignments are intimately linked to shape–filament correlations, which are the primary source of IA in weak‑lensing surveys. Current IA mitigation strategies rely on empirical shape–density correlations and lack a physical grounding in the underlying tidal‑torque theory. By jointly measuring spin–filament and shape–filament alignments, the authors propose to build a predictive IA framework that explicitly depends on galaxy mass, morphology, environment, and redshift. Such a framework would dramatically reduce systematic uncertainties in Stage‑IV weak‑lensing experiments (e.g., Euclid, Rubin, Roman), improving constraints on neutrino masses, the dark‑energy equation of state, and other fundamental cosmological parameters.

The paper outlines why existing facilities cannot achieve these goals. Large‑area multiplexed spectroscopic surveys (DESI, 4MOST, PFS, MOONS) provide high‑quality redshifts but are limited to a few galaxies per square arcminute, far below the 30–50 gal arcmin⁻² surface density required to reconstruct filaments at sub‑Mpc resolution. Integral‑field spectrographs (MUSE, SAMI, MaNGA, Hector, ELT‑HARMONI) deliver resolved kinematics but only for modest samples (10³–10⁴ galaxies) and over limited sky areas, insufficient for statistical studies across mass, environment, and redshift bins. Consequently, no current or planned instrument can simultaneously (i) obtain millions of galaxy redshifts, (ii) sample the galaxy density needed for sub‑Mpc filament mapping out to z ≈ 1, and (iii) provide high‑resolution ionised‑gas and stellar velocity fields.

To overcome these limitations, the authors specify stringent technical requirements: a 12‑m class ground‑based telescope equipped with a wide‑field (≥ 3 deg²) multi‑object spectrograph capable of > 80 % completeness to magnitude 24.5, delivering a surface density of ~40 000 galaxies per square degree. In parallel, a monolithic integral‑field unit with a 3 × 3 arcmin² field of view must capture spatially‑resolved kinematics (both ionised‑gas lines such as