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3.10. The Sunyaev-Zeldovich Effect in Clusters
The Sunyaev-Zeldovich (1972) effect is a perturbation to the spectrum of the cosmic microwave background radiation as it passes through the hot dense intracluster gas. It is caused by inverse Compton scattering of the radiation by the electrons in the cluster gas.
At the long-wavelength side of the background radiation spectrum, the hot gas lowers the brightness temperature seen through the cluster center by the fractional decrement
where T = 2.73 K is the microwave radiation temperature, 0 is the Thomson scattering optical depth through the cluster (0 = T nedl, where T is the Thomson scattering cross section and dl is the distance along the line of sight), Tx is the intracluster gas temperature, and me is the electron mass.
For typical observed rich cluster parameters of Lx ~ 1044h-2 ergs-1, Rc ~ 0.2h-1 Mpc, and kTx 4 keV, the bremsstrahlung relation (Lx ne2 Rc3 Tx0.5, Section 3.1) implies a central gas density of ne 3 × 10-3h1/2 electrons cm-3, thus yielding 0 3 × 10-3h-1/2 [0 = 0.0064ne(cm-3) Rc(kpc)]. Therefore
This temperature decrement remains constant over the cluster core diameter
and decreases at larger separations.
The effect has been detected in observations of some rich, X-ray luminous clusters (e.g., Coma, A665, A2163, A2218, Cl 0016+16) (Birkinshaw et al. 1984; Jones et al. 1993; Wilbanks et al. 1994; Herbig et al. 1995).