Measuring a dynamical aspect of dark matter - the velocity anisotropy  [slides]

Abstract

I will address two completely different things in this rather informal talk.

• The first is related to the "velocity anisotropy" of dark matter, which is a dynamical property of the dark matter haloes. This has been measured in numerical simulations for 25 years, and only now have we found a way to measure this in galaxy clusters. I will explain the idea, and show how we have just used this on Perseus to measure the velocity anisotropy out to the edge of the galaxy cluster.
• The socalled "turn-around radius" is the place where the infall towards clusters is exactly cancelled by the Hubble expansion. That's where the world stands still. This is typically at 3-6 times the virial radius. We found a way to measure this, and I will explain the idea behind. I will also show the first two measurements, and discuss why this could be rather useful.