The study of cosmological radio backgrounds with the Sunyaev-Zel'dovich effect

Emritte, Mohammad Shehzad
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According to the standard model of cosmology, the Universe has evolved from a thermal bath of elementary particles and photons towards one comprising of collapsed structures such as stars, galaxies and clusters of galaxies. The Cosmic Microwave Background (CMB) spectrum and its angular anisotropy across the sky contain information on the physical processes, matter distribution and evolution of the Universe across cosmic time. Primordial spectral distortions of the CMB and its anisotropy can be studied through the inverse comptonization process occuring in cosmic structures, known as the Sunyaev-Zel'dovich e ect (SZE). This present study demonstrates how the SZE can be used to obtain information on the 21 cm background produced between the Dark Ages (DA) and the Epoch of Reionization (EoR), on Non-Planckian (NP) modi cations of the CMB due to plasma frequency at the recombination epoch, and on the anisotropy of the CMB at cluster locations, through the study of the polarization of the SZE. To these aims, a full relativistic approach is employed, that allows us to calculate the spectra of the SZE and its polarization component with high precision, and allows to calculate it for any kind of electron population (thermal or nonthermal plasma), and for an input spectrum that can deviate from the standard black-body spectrum. The SZE-21cm, which is the comptonized spectrum of the modi ed CMB due to physical processes occuring during the DA and the EoR, is calculated for four models of the 21-cm background. A full spectral analysis of the signal is performed and the importance of relativistic e ects are highlighted. The results demonstrate that relativistic e ects are nonzero over the entire frequency spectrum and hence cannot be ignored, particularly for hot clusters. It is found that the amplitude of the SZE-21cm signal is of the order of Jy and is within the reach of the SKA instrument. Clusters with high temperature and optical depth are optimal targets to search for the SZE-21cm signal. The SKA can measure the signal in the frequency interval 75-90 MHz for clusters with temperature higher than 5 keV. Discerning the SZE-21cm from the standard SZE can be achieved using the SKA depending on the 21-cm background model for temperatures > 10 keV. Using CMB spectral data at both low and high frequencies, upper limits (206, 346 and 418 MHz at 1, 2, 3 con dence level) are placed on NP e ects associated with a non-zero plasma frequency at the recombination epoch. The SZENP is derived for a CMB spectrum modi ed due to plasma e ects using these upperlimits and a unique spectral feature is obtained. A peak occures at the plasma frequency in the SZENP independent of cluster parameters and the possibility of measuring the plasma frequency with the SKA and eVLA is shown. Plasma e ects are also investigated on the spectrum of the cosmological 21-cm background and it is found that such an e ect is important to consider when recovering the history of the Universe during these epochs. Polarization is a natural outcome of inverse Compton (IC) scattering and the anisotropy of the CMB plays a big role in the production of polarization in Comptonization process. The SZE polarization associated with the anisotropy of the CMB is derived in the full relativistic regime for any general electron distribution. The spectral shapes of the Stokes parameters induced by the IC scattering of the multipoles of the CMB for thermal and non-thermal electrons are derived, focusing mainly on the quadrupole and octupole which provide the largest possible detectable signals in cosmic structures. Our results demonstrate the implication of relativistic e ects, which become important for high temperature or non-thermal cluster environments. When relativistic e ects are accounted for, all the multipoles of the CMB are involved in the production of polarization. The octupole induced polarization spectrum reveals the existence of a cross-over frequency which is dependent on cluster parameters such as temperature, minimum momentum and spectral index. The possibilities to disentangle the quadrupole spectrum from the octupole one are discussed, which would allow the measurments of these multipoles at cluster locations. The generality of our approach allows us to calculate the SZE polarization spectra of the Bullet cluster using multifrequency SZE data in intensity and compare the results with the sensitivities of the SKA, ALMA, Millimetron and CORE++ instruments. Although the e ects that we studied here are small, however, they are still within the detection limits of the SKA, due to its very high sensitivity. Therefore, the SKA will play a big role in the study of cosmological radio backgrounds by providing high precision SZE data.
A thesis submitted to the Faculty of Science, University of Witwatersrand, in the ful lment of the requirements for the degree of Doctor of Philosophy Johannesburg, South Africa, 2017.
Emritte, Mohammad Shehzad (2017) The study of cosmological radio backgrounds with the Sunyaev-Zel'dovich effect, University of the Witwatersrand, Johannesburg, <>