The Cosmic Ballet II: Spin alignment of galaxies and haloes with
  large-scale filaments in the EAGLE simulation
  We investigate the alignment of galaxies and haloes relative to cosmic web
filaments using the EAGLE hydrodynamical simulation. We identify filaments by
applying the NEXUS+ method to the mass distribution and the Bisous formalism to
the galaxy distribution. Both web finders return similar filamentary structures
that are well aligned and that contain comparable galaxy populations. EAGLE
haloes have an identical spin alignment with filaments as their counterparts in
dark matter only simulations: a complex mass dependent trend with low mass
haloes spinning preferentially parallel to and high mass haloes spinning
preferentially perpendicular to filaments. In contrast, galaxy spins do not
show such a spin transition and have a propensity for perpendicular alignments
at all masses, with the degree of alignment being largest for massive galaxies.
This result is valid for both NEXUS+ and Bisous filaments. When splitting by
morphology, we find that elliptical galaxies show a stronger orthogonal
spin--filament alignment than spiral galaxies of similar mass. The same is true
of their haloes, with the host haloes of elliptical galaxies having a larger
degree of orthogonal alignment than the host haloes of spirals. Due to the
misalignment between galaxy shape and spin, galaxy minor axes are oriented
differently with filaments than galaxy spins. We find that the galaxies whose
minor axis is perpendicular to a filament are much better aligned with their
host haloes. This suggests that many of the same physical processes determine
both the galaxy--filament and the galaxy--halo alignments.
