Velocity Anisotropy and Shape Bias in the Caustic Technique
  We use the Millennium Simulation to quantify the statistical accuracy and
precision of the escape velocity technique for measuring cluster-sized halo
masses at z~0.1. We show that in 3D, one can measure nearly unbiased (<4%) halo
masses (>1.5x10^14 M_solar h^-1) with 10-15% scatter. Line-of-sight projection
effects increase the scatter to ~25%, where we include the known velocity
anisotropies. The classical "caustic" technique incorporates a calibration
factor which is determined from N-body simulations. We derive and test a new
implementation which eliminates the need for calibration and utilizes only the
observables: the galaxy velocities with respect to the cluster mean v, the
projected positions r_p, an estimate of the Navarro-Frenk-White (NFW) density
concentration and an estimate of the velocity anisotropies, beta. We find that
differences between the potential and density NFW concentrations induce a 10%
bias in the caustic masses. We also find that large (100%) systematic errors in
the observed ensemble average velocity anisotropies and concentrations
translate to small (5%-10%) biases in the inferred masses.
