Where Galaxies Point: First Measurement of the Large-Scale Axial Intrinsic Alignment

Abstract

We report evidence for large-scale axial intrinsic alignment (LAIA): a coherent axis shared by galaxies and cosmic-web filaments. Applying an orientation-field estimator to Dark Energy Survey (DES) Y3 shape data, we identify a preferred axis in galaxy orientations. Ellipticals' semi-major and spirals' semi-minor axes align with it, producing a $4.7σ$ signal whose pattern and amplitude hierarchy are consistent with morphology-dependent tidal-alignment and tidal-torquing expectations. Independently, Sloan Digital Sky Survey (SDSS) filament catalogues yield a compatible axis: northern and southern Galactic samples agree within $\simeq1σ$, the combined signal reaches $12.6σ$, and the axis lies within $\simeq2σ$ of the high-redshift galaxy sample direction. Because DES and SDSS footprints overlap marginally, this agreement is unlikely to arise from direct galaxy--filament alignment. It therefore provides a multi-survey, multi-observable test of a large-scale orientation field, stable under redshift and systematics tests. $N$-body mocks based on an isotropic $Λ$CDM cosmology with standard intrinsic-alignment prescriptions, including Euclid Flagship 2 and MICECAT v2, do not reproduce the pattern. LAIA provides a new statistical-isotropy probe linking galaxy morphology, cosmic-web structure and large-scale tidal fields.