Cosmic vorticity and the origin of halo spins
  In the standard model of cosmology, structure emerges out of non-rotational
flow and the angular momentum of collapsing halos is induced by tidal torques.
The growth of halo angular momentum in the linear and quasi-linear phases is
associated with a shear, curl-free, flow and it is well described within the
linear framework of tidal torque theory (TTT). However, TTT is rendered
irrelevant as haloes approach turn around and virialization. At that stage the
flow field around halos has non-zero vorticity. Using a cosmological
simulation, we have examined the importance of the curl of the velocity field
(vorticity) in determining halo spin, finding a strong alignment between the
two. We have also examined the alignment of vorticity with the principle axes
of the shear tensor, finding that it tends to be perpendicular to the axis
along which material is collapsing fastest (e1). This behavior is independent
of halo masses and cosmic web environment. Our results agree with previous
findings on the tendency of halo spin to be perpendicular to e1, and of the
spin of (simulated) halos and (observed) galaxies to be aligned with the
large-scale structure. Our results imply that angular momentum growth proceeds
in two distinct phases. In the first phase angular momentum emerges out of a
shear, curl-free, potential flow, as described by TTT. In the second phase, in
which haloes approach virialization, the angular momentum emerges out of a
vortical flow and halo spin becomes strongly aligned with the vorticity of the
ambient flow field.
