Flow Patterns around Dark Matter Halos: the Link between Halo Dynamical
  Properties and Large Scale Tidal Field
  We study how halo intrinsic dynamical properties are linked to their
formation processes for halos in two mass ranges, $10^{12}-10^{12.5}h^{-1}{\rm
M_\odot}$ and $\ge 10^{13}h^{-1}{\rm M_\odot}$, and how both are correlated
with the large scale tidal field within which the halos reside at present. Halo
merger trees obtained from cosmological $N$-body simulations are used to
identify infall halos that are about to merge with their hosts. We find that
the tangential component of the infall velocity increases significantly with
the strength of the local tidal field, but no strong correlation is found for
the radial component. These results can be used to explain how the internal
velocity anisotropy and spin of halos depend on environment. The position
vectors and velocities of infall halos are aligned with the principal axes of
the local tidal field, and the alignment depends on the strength of the tidal
field. Opposite accretion patterns are found in weak and strong tidal fields,
in the sense that in a weak field the accretion flow is dominated by radial
motion within the local structure, while a large tangential component is
present in a strong field. These findings can be used to understand the strong
alignments we find between the principal axes of the internal velocity
ellipsoids of halos and the local tidal field, and their dependence on the
strength of tidal field. They also explain why halo spin increases with the
strength of local tidal field, but only in weak tidal fields does the
spin-tidal field alignment follow the prediction of the tidal torque theory. We
discuss how our results may be used to understand the spins of disk galaxies
and velocity structures of elliptical galaxies and their correlations with
large-scale structure.
