Friday, January 20, 2012


in the thermal Sunyaev-Zel'dovich effect there's an interaction between a galaxy cluster's electrons and the CMB photons. Does a galaxy cluster become colder in time due to the thermal SZ-effect?

for bonus points: does a cluster get slowed down by the kinetic SZ-effect? if yes, relative to which frame of reference?


  1. the thermal SZ-effect is in fact tapping thermal energy from the ICM of a cluster... but whether the cluster becomes colder or not depends on the time scale of this energy tapping. A cluster of galaxies is a self-gravitating object and has a negative specific heat if you remove thermal energy on a long time-scale, longer than the dynamical time-scale of the cluster, and a normal positive specific heat if you remove the energy fast. because the tapping of thermal energy by the thermal SZ-effect is so weak, the cluster in fact heats up (very, very slowly).

    bonus points: the kinetic SZ-effect slows down the cluster until it is at rest in a frame in which the CMB is isotropic.

  2. what I always find fascinating about the SZ-effect and what illustrates the fact that you only need isotropic and not thermal velocities for the thermal SZ-effect is this: if you've got turbulent motion inside the cluster, it would show up as kinetic SZ-effect if it is resolved and thermal SZ if it is unresolved!