Clusters of Galaxies

In addition to the primary CMB anisotropies generated in the early Universe, secondary anisotropies can be generated between the epoch of decoupling and the present epoch. The most important of these are caused by the Sunyaev-Zeldovich effect (Zeldovich & Sunyaev 1969) in which CMB photons are scattered by free electrons in the hot gaseous component of rich clusters of galaxies. The temperature of the CMB in the direction of a rich cluster of galaxies is modified in a way which depends on both the spatial distribution and temperature of the hot cluster gas, and the relative velocity of the cluster with respect to the rest frame defined by the CMB. The Sunyaev-Zeldovich (hereafter SZ) effect has been observed unambiguously in a small number of rich clusters in ground based observations (see e.g. Rephaeli 1995, Birkinshaw 1998, for recent comprehensive reviews). However, as we show in this Section, the high angular resolution and sensitivity of PLANCK should lead to the detection of the SZ effect in several thousand rich clusters of galaxies and to valuable statistical information on cluster peculiar motions.