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Astron. Astrophys. 360, 417-430 (2000)

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Comptonization of the cosmic microwave background by relativistic plasma

T.A. Enßlin and C.R. Kaiser

Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85740 Garching, Germany

Received 26 January 2000 / Accepted 20 June 2000


We investigate the spectral distortion of the cosmic microwave background (CMB) caused by relativistic plasma. Within the Thomson regime, an exact analytic expression for the photon scattering kernel of a momentum power-law electron distribution is given, which is valid from the non- to the ultra-relativistic regime. The decrement in the photon spectrum saturates for electron momenta above [FORMULA] to that of an optically thick absorber with the optical depth of the relativistic electrons given by the Thomson limit. Thus, the ultra-relativistic Sunyaev-Zeldovich (SZ) decrement measures the electron number and not the energy content. On the other hand, the relativistic SZ increment at higher frequencies depends strongly on the spectral shape of the electrons, allowing for investigation of relativistic electron populations with future instruments.

We calculate the expected Comptonization due to the energy release of radio galaxies, which we estimate to be [FORMULA]. We investigate Comptonization from (a) the part of the released energy which is thermalized and (b) the relativistic, remnant radio plasma, which may form a second, relativistic phase in the intergalactic medium, nearly unobservable for present day instruments (presence of so called `radio ghosts'). We find a thermal Comptonization parameter due to (a) of [FORMULA] and (b) an optical depth of relativistic electrons in old radio plasma of [FORMULA]. If a substantial fraction of the volume of clusters of galaxies is filled with such old radio plasma the SZ effect based determination of the Hubble constant is biased to lower values, if this is not accounted for. Finally, it is shown that a supra-thermal population of electrons in the Coma cluster would produce a signature in the Wien-tail of the CMB, which is marginally detectable with a multifrequency measurement by the Planck satellite. Such an electron population is expected to exist, since its bremsstrahlung would explain Coma's recently reported high energy X-ray excess.

Key words: cosmology: cosmic microwave background – radiation mechanisms: non-thermal – scattering – galaxies: intergalactic medium – galaxies: active – galaxies: clusters: individual: Coma

Send offprint requests to: T.A.E.

Correspondence to: T.A.E., (ensslin@mpa-garching.mpg.de) and C.R.K., (kaiser@mpa-garching.mpg.de)

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© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000