Quantifying the collisionless nature of dark matter and galaxies in
  A1689
  We use extensive measurements of the cluster A1689 to assess the expected
similarity in the dynamics of galaxies and dark matter (DM) in their motion as
collisionless `particles' in the cluster gravitational potential. To do so we
derive the radial profile of the specific kinetic energy of the cluster
galaxies from the Jeans equation and observational data. Assuming that the
specific kinetic energies of galaxies and DM are roughly equal, we obtain the
mean value of the DM velocity anisotropy parameter, and the DM density profile.
Since this deduced profile has a scale radius that is higher than inferred from
lensing observations, we tested the validity of the assumption by repeating the
analysis using results of simulations for the profile of the DM velocity
anisotropy. Results of both analyses indicate a significant difference between
the kinematics of galaxies and DM within $r \lesssim 0.3r_{\rm vir}$. This
finding is reflected also in the shape of the galaxy number density profile,
which flattens markedly with respect to the steadily rising DM profile at small
radii. Thus, $r \sim 0.3r_{\rm vir}$ seems to be a transition region interior
to which collisional effects significantly modify the dynamical properties of
the galaxy population with respect to those of DM in A1689
