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The Sunyaev-Zel'dovich (SZ) effect is the name given to the process by which the CMB blackbody spectrum is distorted by the presence of galaxy clusters. Galaxy clusters are the largest gravitationally-collapsed (virialized) structures in the universe, whose abundance and structure provide powerful probes of cosmology. Gas falling into the gravitational potential well of these clusters is heated to roughly 10^8 Kelvin and becomes ionized. Photons from the CMB pass through this ionized plasma, and as many as 1-2% of the CMB photons can be inverse Compton (Thompson) scattered by the hot gas. On average the energy of the scattered photons is increased, spectrally distorting the CMB in a characteric manner that is known as the thermal SZ effect. Deviations in intensity from the black-body ideal of the CMB are shown, as a function of frequency, as the blue line in the lower right graph. The intensity spectrum of the CMB is shown for comparison as a black dotted line, scaled by 5e-4. (The observing frequencies of the APEX SZ Survey, 150 GHz and 217 GHz, correspond to the decrement region and null region of the thermal SZ effect, respectively.)
The kinetic SZ effect is caused by the motion of of the galaxy clusters with respect to the rest frame of the CMB. The net motion of the scattering electrons in the hot intercluster gas imparts a Doppler shift to the scattered photons. This kinetic SZ effect is shown in the graph as the red dotted line. It is a decrement in overall power because the galaxy is moving away from us. A net increment in power can also occur if the galaxy is moving toward us. This effect in principle allows one to determine the peculiar velocities of the galaxy clusters; peculiar meaning 'with respect to the CMB rest frame'.