As CMB photons travel from the surface of last scattering to the observer, secondary anisotropies can arise due to the interaction of the CMB photons with intervening matter. Of particular interest is the S-Z effect , which occurs when CMB photons travel through a cluster of galaxies. Compton scattering of CMB photons by electrons in this intra-cluster plasma can result in an optical depth as high as 0.02, resulting in a distortion of the CMB spectrum at the mK level. There are multiple components of the S-Z effect which result from distinct velocity components of the scattering electrons. The spectral distortions produced by components of the S-Z effect are shown here.
Solid curves indicate increments in the CMB temperature, and the dashed curves decrements.
Accurate arcminute-resolution maps at a variety of millimeter and submillimeter wavelengths would provide information about the internal density, temperature, and velocity structure of cluster gas throughout the Hubble volume. The observed surface brightness difference of both the thermal and kinematic components is independent of the cluster redshift, as long as the cluster is resolved. So far, there has not been any detection of the kinematic S-Z effect. Only recently has a system sensitive enough to measure the small kinematic signal been built. While this experiment represents a considerable step forward, the results on Mauna Kea have been limited by sky noise.