An observed link between spin-filament alignment flips and bulge formation

The observational study of the interplay between galaxy angular momentum and structure in the cosmic web is challenging due to the weakness of the signal. We study the alignments of galaxy spin axes with respect to cosmic web filaments as a function of different properties for galaxies and for their bulge and disk components. We exploit the SAMI Galaxy Survey to identify 3D spin axes from spatially-resolved kinematics and to decompose galaxies into their kinematic bulge and disk components. We use the GAMA spectroscopic survey to reconstruct the surrounding cosmic filaments. We find a strong correlation between the galaxy spin-filament alignment and the mass of the bulge: galaxies with lower bulge masses tend to have their spins parallel to the closest filament, while high-bulge mass galaxies show a perpendicular orientation. This observed link between the flip in the spin-filament alignment and the growth of the bulge can be explained by mergers. Bulges tend to have perpendicular alignments, indicating mergers as their main formation channel; in contrast, pseudo-bulges tend to have a parallel alignment, consistent with secular accretion. Disks show different alignments according to their kinematic features and bulge mass, suggesting varying formation pathways. We conclude that bulge mass is the primary parameter tracing the processes that cause the galaxy spin-filament alignment to flip from parallel to perpendicular.