Estimating the dark matter velocity anisotropy to the cluster edge
  Dark matter dominates the properties of large cosmological structures such as
galaxy clusters, and the mass profiles of the dark matter have been measured
for these equilibrated structures for years using X-rays, lensing or galaxy
velocities. A new method has been proposed, which should allow us to estimate a
dynamical property of the dark matter, namely the velocity anisotropy. For the
gas a similar velocity anisotropy is zero due to frequent collisions, however,
the collisionless nature of dark matter allows it to be non-trivial. Numerical
simulations have for years found non-zero and radially varying dark matter
velocity anisotropies. Here we employ the method proposed by Hansen and
Pifaretti (2007), and developed by Host et al. (2009) to estimate the dark
matter velocity anisotropy in the bright galaxy cluster Perseus, to near 5
times the radii previously obtained. We find the dark matter velocity
anisotropy to be consistent with the results of numerical simulations, however,
still with large error-bars. At half the virial radius we find the velocity
anisotropy to be non-zero at 1.7 standard deviations, lending support to the
collisionless nature of dark matter.
