Abstract

Contributed Talk - Plenary

Alignment of spiral and elliptical galaxies with the cosmic web

Yuvraj Muralichandran, Noam I. Libeskind, Elmo Tempel
Department of Physics and Astronomy, University of Potsdam, Germany; Leibniz-Institut für Astrophysik Potsdam (AIP), Germany; Tartu Observatory, University of Tartu, Estonia

The angular momentum of galaxies is expected to originate from tidal torques during the early stages of structure formation, leading to correlations between galaxy spins and the surrounding large-scale structure. In this work, we investigate the alignment between galaxy spin orientations and filamentary structures of the cosmic web using the Siena Galaxy Atlas and filaments identified with the Bisous process applied to SDSS DR12. The analysis is restricted to galaxies within 2 Mpc of filament spines, comprising over 32,000 spiral and 18,000 elliptical systems. Galaxy spin directions are reconstructed from projected shapes using inclination modelling, and alignment is quantified through the distribution of the cosine of the angle between the spin vector and the filament axis, with statistical significance assessed via Monte Carlo randomisation. We find a clear morphological dependence in the alignment signal. Elliptical galaxies exhibit a statistically significant tendency for their spin axes to be oriented perpendicular to filament spines, with significance reaching several sigma depending on the estimator. In contrast, spiral galaxies show only a weak alignment signal, with indications of a parallel tendency emerging primarily in restricted subsamples. The strength of the alignment is found to depend on both filament proximity and galaxy luminosity, with the most pronounced signals arising for brighter ellipticals and for spirals at intermediate distances from the filament spine. Using Bayesian optimisation, we identify subsets that maximise the significance of the signal, although the spiral alignment remains marginal. These results provide observational support for the mass- and morphology-dependent spin–filament alignment predicted by tidal torque theory and numerical simulations, while also highlighting the challenges imposed by projection effects and intrinsic degeneracies in recovering galaxy spin orientations.