And yet it flips: connecting galactic spin and the cosmic web
  We study the spin alignment of galaxies and halos with respect to filaments
and walls of the cosmic web, identified with DisPerSE, using the SIMBA
simulation from z=0-2. Massive halos' spins are oriented perpendicularly to
their closest filament's axis and walls, while low mass halos tend to have
their spins parallel to filaments and in the plane of walls. A similar
mass-dependent spin flip is found for galaxies, albeit with a weaker signal
particularly at low mass and low-z, suggesting that galaxies' spins retain
memory of their larger-scale environment. Low-z star-forming and
rotation-dominated galaxies tend to have spins parallel to nearby filaments,
while quiescent and dispersion-dominated galaxies show preferentially
perpendicular orientation; the star formation trend can be fully explained by
the stellar mass correlation, but the morphology trend cannot. There is a
strong dependence on HI mass, such that high-HI galaxies tend to have parallel
spins while low-HI galaxies are perpendicular, which persists even when
matching samples in stellar mass, suggesting that HI content traces anisotropic
infall more faithfully than the stellar component. Finally, at fixed stellar
mass, the strength of spin alignments correlates with the filament's density,
with parallel alignment for galaxies in high density environments. These
findings are consistent with conditional tidal torque theory, and highlight a
significant correlation between galactic spin and the larger scale tides that
are important e.g. for interpreting weak lensing studies. SIMBA allows us to
rule out numerical grid locking as the cause of previously-seen low mass
alignment.
