The SunyaevâÂÂZeldovich effect (named after Rashid Sunyaev and Yakov B. Zeldovich and often abbreviated as the SZ effect) is the spectral distortion of the cosmic microwave background (CMB) through inverse Compton scattering by high-energy electrons in galaxy clusters, in which the low-energy CMB photons receive an average energy boost during collision with the high-energy cluster electrons. Observed distortions of the cosmic microwave background spectrum are used to detect the disturbance of density in the universe. Using the SunyaevâÂÂZeldovich effect, dense clusters of galaxies have been observed.
The SunyaevâÂÂZeldovich effect was predicted by Rashid Sunyaev and Yakov Zeldovich to describe anisotropies in the CMB. The effect is caused by the CMB interacting with high energy electrons. These high energy electrons cause inverse Compton scattering of CMB photons which causes a distortion in the radiation spectrum of the CMB. The SunyaevâÂÂZeldovich effect is most apparent when observing galactic clusters. Analysis of CMB data at higher angular resolution (high -values) requires taking into account the SunyaevâÂÂZeldovich effect.
The SunyaevâÂÂZeldovich effect can be divided into different types:
The SunyaevâÂÂZeldovich effect is of major astrophysical and cosmological interest. It can help determine the value of the Hubble constant, determine the location of new galaxy clusters, and in the study of cluster structure and mass. Since the SunyaevâÂÂZeldovich effect is a scattering effect, its magnitude is independent of redshift, which means that clusters at high redshift can be detected just as easily as those at low redshift.
The distortion of the CMB resulting from a large number of high energy electrons is known as the thermal SunyaevâÂÂZeldovich effect. The thermal SunyaevâÂÂZeldovich effect is most commonly studied in galaxy clusters. By comparing the SunyaevâÂÂZeldovich effect and X-ray emission data, the thermal structure of the cluster can be studied, and if the temperature profile is known, SunyaevâÂÂZeldovich data can be used to determine the baryonic mass of the cluster along the line of sight. Comparing SunyaevâÂÂZeldovich and X-ray data can also be used to determine the Hubble constant using the angular diameter distance of the cluster. These thermal distortions can also be measured in superclusters and in gases in the local group, although they are less significant and more difficult to detect. In superclusters, the effect is not strong (< 8 üK), but with precise enough equipment, measuring this distortion can give a glimpse into large-scale structure formation. Gases in the local group may also cause anisotropies in the CMB due to the thermal SunyaevâÂÂZeldovich effect which must be taken into account when measuring the CMB for certain angular scales.
The kinematic SunyaevâÂÂZeldovich effect is caused when a galaxy cluster is moving relative to the Hubble flow. The kinematic SunyaevâÂÂZeldovich effect gives a method for calculating the peculiar velocity:
where is the peculiar velocity, and is the optical depth. In order to use this equation, the thermal and kinematic effects need to be separated. The effect is relatively weak for most galaxy clusters. Using gravitational lensing, the peculiar velocity can be used to determine other velocity components for a specific galaxy cluster. These kinematic effects can be used to determine the Hubble constant and the behavior of clusters.
Current research is focused on modelling how the effect is generated by the intracluster plasma in galaxy clusters, and on using the effect to estimate the Hubble constant and to separate different components in the angular average statistics of fluctuations in the background. Hydrodynamic structure formation simulations are being studied to gain data on thermal and kinetic effects in the theory. Observations are difficult due to the small amplitude of the effect and to confusion with experimental error and other sources of CMB temperature fluctuations. To distinguish the SZ effect due to galaxy clusters from ordinary density perturbations, both the spectral dependence and the spatial dependence of fluctuations in the cosmic microwave background are used.
A factor which facilitates high redshift cluster detection is the angular scale versus redshift relation: it changes little between redshifts of 0.3 and 2, meaning that clusters between these redshifts have similar sizes on the sky. The use of surveys of clusters detected by their SunyaevâÂÂZeldovich effect for the determination of cosmological parameters has been demonstrated by Barbosa et al. (1996). This might help in understanding the dynamics of dark energy in surveys (South Pole Telescope, Atacama Cosmology Telescope, Planck).
In 1984, researchers from the Cambridge Radio Astronomy Group and the Owens Valley Radio Observatory first detected the SunyaevâÂÂZeldovich effect from clusters of galaxies. Ten years later, the Ryle Telescope was used to image a cluster of galaxies in the SunyaevâÂÂZeldovich effect for the first time.
In 1987 the Cosmic Background Explorer (COBE) satellite observed the CMB and gave more accurate data for anisotropies in the CMB, allowing for more accurate analysis of the SunyaevâÂÂZeldovich effect.
Instruments built specifically to study the effect include the SunyaevâÂÂZeldovich camera on the Atacama Pathfinder Experiment, and the SunyaevâÂÂZeldovich Array, which both saw first light in 2005. In 2012, the Atacama Cosmology Telescope (ACT) performed the first statistical detection of the kinematic SZ effect. In 2012 the kinematic SZ effect was detected in an individual object for the first time in MACS J0717.5+3745.
As of 2015, the South Pole Telescope (SPT) had used the SunyaevâÂÂZeldovich effect to discover 415 galaxy clusters. The SunyaevâÂÂZeldovich effect has been and will continue to be an important tool in discovering hundreds of galaxy clusters.
Recent experiments such as the OLIMPO balloon-borne telescope try to collect data in specific frequency bands and specific regions of the sky in order to pinpoint the SunyaevâÂÂZeldovich effect and give a more accurate map of certain regions of the sky.