The Sunyaev-Zeldovich effect
The Sunyaev-Zeldovich effect (SZ effect) has been predicted by Sunyaev &
Zeldovich (1969, 1972, 1980):
They showed, that the passage of the cosmic microwave background radiation (CMB)
through a hot electron cloud would distort the spectrum of the radiation. Due to
the high energy of the electrons, and the homogeneity and isotropy of the CMB,
the photons gain energy. The change of the spectrum due to the Compton
interaction between the photons and the electrons is shown in Fig.1. There one
can see the orginal spectrum of the CMB (a Planck spectrum with the temperature
of 2.73 Kelvin, solid line) and the spectrum distorted due to the interaction
(dashed line).
Fig.1
Sunyaev & Zeldovich interpreted this change with modifications of the
brightness temperature TR of the CMB. It is known that the intensity
in a Planck spectrum will be lower at each frequency if the temperature will be
lower and vice versa. In Fig.2 this fact is shown for three temperatures.
Fig.2
If one combines the two plots one can see: in the Rayleigh-Jeans regime (at low
frequencies) the change of the spectrum due to the Compton interaction can also
be produced with a lower temperature of the radiation. In the Wien part of the
spectrum (at high frequencies) one can interpret the change with a higher
radiation temperature.
This is the predication of the SZ effect:
If one measures the brightness temperature TR of the CMB towards a
region which contains hot electrons, one expects a decrement of this temperature
at low frequencies and an increment at high frequencies compared to the normal
brightness temperature (2.73 Kelvin). The expected temperature distribution
looks qualitatively like plot Fig.3.
Fig.3
X-ray observations of clusters of galaxies show so-called intracluster gas with
high temperatures (~107...8 Kelvin). At these temperatures the gas is
completely ionized, so there are free hot electrons.
Observations of the CMB with radio telescopes (low frequencies) towards clusters
of galaxies should show a diminution of the brightness temperature. The value
depends on the properties of the intracluster gas (temperature, density, gas
distribution), of course and is very small (~0.1...1*10-3 Kelvin).
The SZ effect is of extreme astrophysical and cosmological interest. Firstly,
the detection of the SZ effect verifies the cosmological origin of the CMB.
Furthermore, combining the radio with X-ray observations of the measured cluster
allows one to determine the distance of the cluster, and so to determine the
value of the famous Hubble constant H0 and, in case of very distant
clusters, the value of the deceleration parameter q0 (see e.g.
Inagaki et al. 1995).
Inagaki Y., Suginohara T., Suto Y., 1995, Publ. Astron. Soc. Japan 47, 411
Sunyaev, R., Zeldovich, Ya.B. 1972, Comm. Astr. Space Phys. 4, 173
Sunyaev, R., Zeldovich, Ya.B. 1980, Ann. Rev. Astron. Astrophys. 18, 537
Zeldovich, Ya.B.,Sunyaev, R., 1969, Astrophys. Space Sci. 4, 301