Sunyaev-Zel'dovich effect


Sunyaev–Zel'dovich effect

(sûn-yah -yeff zyel -dŏ-vich) (S–Z effect) The apparent change in temperature of the microwave background in the direction of very hot (108 K) plasma clouds, such as those around rich clusters of galaxies. It is caused by inverse Compton scattering of microwave background photons from highly energetic electrons in the plasma. When looked at with a radio telescope, the number of radio-frequency photons is depleted so the radiation appears cooler. The effect shows that the photons truly originated from behind the cluster (i.e. in the background) and can also be used to determine the distance to the cluster. The value of the Hubble constant can be measured by comparison of the X-ray luminosity of the galaxy to its microwave decrement.