The study of spin-filament alignments to unravel the evolution of galaxies and their components

The study of the interplay between galaxy angular momentum and structures in the cosmic web is a powerful tool to constrain galaxy evolution scenarios. We study the alignments of galaxies' spin axes with respect to nearby cosmic web filaments as a function of various properties of the galaxies and their constituent bulges and discs. We exploit the SAMI Galaxy Survey to identify 3D spin axes from spatially-resolved kinematics and to decompose galaxies into their kinematic bulge and disc components. The GAMA redshift survey is used to reconstruct the surrounding cosmic filaments. The mass of the bulge is the primary parameter of correlation with spin-filament alignments: galaxies with lower bulge masses tend to have their spins parallel to the closest filament, while galaxies with higher bulge masses are more perpendicularly aligned. Other galaxy properties, such as visual morphology, stellar age, star formation activity, kinematic parameters and local environment, show secondary correlations. The observational link between bulge growth and flipping of spin-filament alignments from parallel to perpendicular can be explained by mergers, which can drive both alignment flips and bulge formation. The separate study of bulge and disc spin-filament alignments reveals additional clues about the formation pathways of these galaxy components: bulges tend to have more perpendicular alignments, suggesting they are merger products, while discs show different tendencies according to their kinematic features and the mass of the associated bulge, pointing to multiple formation scenarios.

Finally, we investigate the impact of central supermassive black holes on galaxy spin-filament alignments, considering stellar spins and ionised gas spins separately. Galaxies characterised by an active galactic nucleus (AGN) are explored to understand the role of instantaneous black hole activity. Central stellar velocity dispersion is used as a proxy for black hole mass to disentangle the impact of integrated black hole activity. We find evidence for the ionised gas spin–filament alignments to be influenced by AGN, while for the stellar alignments a significant role is played only by the integrated black hole activity. These effects are secondary: bulge mass remains the primary parameter of correlation for both stellar and ionised gas spin-filaments alignments.

Our findings highlight the importance of integral field spectroscopy surveys for elucidating how changes in galaxy angular momentum within the cosmic web are related to the evolution of galaxies and their components.