Anisotropic Sunyaev-Zel'dovich Effect

Abstract

The spectral distortion in the Cosmic Microwave Background Radiation (CMBR) caused from the inverse Compton scattering of low energy CMB photons by thermally distributed hot electrons in massive clusters of galaxies - better known as the thermal Sunyaev-Zel'dovich effect is usually estimated by the analytical or semi-analytical solutions of Kompaneets equation. These solutions are based on the assumption that the scattering is isotropic, i.e., the intensity of the CMBR remains angle-independent after scattering. In this letter, I present the solution of the full radiative transfer equations by incorporating anisotropic inverse Compton scattering of CMBR photons by relativistic electrons and show that the thermal Sunyaev-Zel'dovich effect is strongly angle-dependent. The spectral distortion in the CMBR alters significantly in different angular directions. The amount of distortion increases several times when the angle between the axis of symmetry and the photon ray path is increased. It is pointed out that for anisotropic scattering the Compton y parameter is angle dependent. Hence it is important to consider the direction of the emergent radiation with respect to the axis of symmetry in the electron rest frame while estimating any cosmological quantity from the Sunyaev-Zel'dovich effect. ~