A kinematic classification of the cosmic web
  A new approach for the classification of the cosmic web is presented. In
extension of the previous work of Hahn et al. (2007) and Forero-Romero et al.
(2009) the new algorithm is based on the analysis of the velocity shear tensor
rather than the gravitational tidal tensor. The procedure consists of the
construction of the the shear tensor at each (grid) point in space and the
evaluation of its three eigenvectors. A given point is classified to be either
a void, sheet, filament or a knot according to the number of eigenvalues above
a certain threshold, 0, 1, 2, or 3 respectively. The threshold is treated as a
free parameter that defines the web. The algorithm has been applied to a dark
matter only, high resolution simulation of a box of side-length 64$h^{-1}$Mpc
and N = $1024^3$ particles with the framework of the WMAP5/LCDM model. The
resulting velocity based cosmic web resolves structures down to <0.1$h^{-1}$Mpc
scales, as opposed to the ~1$h^{-1}$Mpc scale of the tidal based web. The
under-dense regions are made of extended voids bisected by planar sheets, whose
density is also below the mean. The over-dense regions are vastly dominated by
the linear filaments and knots. The resolution achieved by the velocity based
cosmic web provides a platform for studying the formation of halos and galaxies
within the framework of the cosmic web.
