A Sunyaev-Zel'dovich Effect Primer

The Sunyaev-Zel'dovich Effect (SZE) is a signature left in the cosmic microwave background (CMB) by collapsed structures containing baryons. This signature results from inverse Compton interactions between hot electrons in the collapsed structures and the low energy photons in the CMB. These interactions drive the population of electrons and photons toward equilibrium, resulting in a net transfer of energy from the electrons to the photons. In galaxy clusters the typical optical depth to Compton scattering is ~1%, so only 1 out of every 100 CMB photons that passes through the cluster suffers from these interactions. Therefore, rather than driving the electron and CMB photon populations all the way to equilibrium, the SZE results in a small distortion of the CMB spectrum.

Amplitude of SZE Distortion versus Frequency

Here we plot the antennae temperature (in microKelvin) of the SZE distortion versus frequency. For frequencies below ~200GHz the distortion results in an effective decrease of the CMB temperature, whereas above ~200 GHz the distortion mimics a temperature enhancement. The red line is the non-relativistic distortion of a high mass cluster with a Compton y parameter of 10^-4, and the green corresponds to the distortion of a lower mass clusters with a Compton y parameter of 5x10^-5. The blue line marks the 26-36 GHz band over which the SZE Imaging Experiment receivers are sensitive.

A typical SZE decrement at 30GHz for a massive cluster is ~1 mK, or 1 thousandth of a Kelvin. The CMB temperature is 2.728 Kelvin, so the SZE decrement for galaxy clusters is a few parts in 10,000, making it a challenging observable. Interferometric observations offer some advantages over single dish observations when it comes to making this precise measurement. For a discussion of these advantages and other SZE facts please refer to John Carlstrom's contribution to the Nobel Symposium on Particle Physics and the Universe: "Imaging the Sunyaev-Zel'dovich Effect".