Detection of the Vorticity Effect on the Disk Orientations
  We present an observational evidence for the vorticity effect on the
nonlinear evolution of the galaxy angular momentum. We first calculate the
vorticity as the curl of the peculiar velocity field reconstructed from the
2MASS redshift survey. Then, measuring the alignments between the vorticity and
the tidal shear fields, we study how the alignment strength and tendency
depends on the cosmic web environment. It is found that in the knot and
filament regions the vorticity vectors are anti-aligned with the directions of
the maximal volume compression while in the void and sheet regions they are
anti-aligned with the directions of the minimal compression. Determining the
spin axes of the nearby large face-on and edge-on disk galaxies from the
Seventh Data Release of the Sloan Digital Sky Survey and measuring their
correlations with the vorticity vectors at the galaxy positions, we finally
detect a clear signal of the alignments between the galaxy spin and the local
vorticity fields. The null hypothesis that there is no alignment between them
is rejected at the 99.999% confidence level. Our result supports
observationally the recently proposed scenario that although the galaxy angular
momentum originates from the initial tidal interaction in the linear regime its
subsequent evolution is driven primarily by the cosmic vorticity field.
