460 Numerical simulations in cosmology
Figure 15.17.Correlation of the characteristic densityρ 0 and radiusr 0 for the dwarf and
LSB galaxies (full and open circles) and for DM halos (crosses) in different cosmological
models. The halos are consistent with observational data: smaller halos are denser. (After
Kravtsovet al1999.)
15.4.3.4 Velocity anisotropy
Inside a large halo, sub-halos or DM particles do not move on either circular or
radial orbits. A velocity ellipsoid can be measured at each position inside a halo. It
can be characterized by an anisotropy parameter defined asβ(r)= 1 −V⊥^2 / 2 Vr^2.
HereV⊥^2 is the velocity dispersion perpendicular to the radial direction andVr^2
is the radial velocity dispersion. For pure radial motionsβ=1. For isotropic
velocitiesβ = 0. The functionβ(r)was estimated for halos in different
cosmological models (see Col ́ınet al1999 for references). By studying 12 rich
clusters with many sub-halos inside each of them, Col ́ınet al(1999) found that
both the sub-halos and DM particles can be described by the same anisotropy