Studying galaxy clusters through X-rays and the Sunyaev-Zel’dovich effect

Abstract

Clusters of galaxies are the largest gravitationally-bound objects in the Universe. Thereby clusters are ideal tracers of cosmic expansion and structure formation that allow tight constraints on the average cosmic density of matter and of other fundamental cosmological parameters. Galaxy clusters can be observed over a broad spectral range, in emission from radio wavelengths to X-rays, through their distortion of the cosmic microwave background radiation (the Sunyaev-Zel'dovich effect), or through their gravitational lensing distortion in the optical appearance of background galaxies. These observables allow detailed measurements of the cluster galaxy populations, of the hot intra-cluster gas, and of the gravitationally dominant dark matter. <br /> This work studies the detection capability and the accuracy of physical constraints derived from present and future X-ray surveys of galaxy clusters. Such imaging-spectroscopy X-ray surveys produce large samples of clusters that allow accurate measures of cosmic structure formation and cosmological parameters only if selection effects are well understood, and the masses of galaxy clusters can be well quantified out to the most distant, i.e. cosmologically earliest, highest-redshift clusters. <br /> In the first part of the thesis, the detection effciency of galaxy clusters for the upcoming eROSITA mission is investigated by means of extensive and dedicated Monte Carlo simulations. Employing a state-of-the-art source detection technique, we determine a cluster detection effciency based on the cluster fluxes and sizes. Using this eROSITA cluster selection function, we found that eROSITA will be able to detect a total of ~1.36x10^5 clusters in the whole sky. This cluster number will allow eROSITA to put stringent constraints on the dark matter and dark energy models. <br /> In the second part of the thesis, the galaxy group detection capability of the future ATHENA observatory is investigated. The detection of the thermal X-ray emission from the faint and small galaxy groups (M_500=5x10^13 M_Sun) at high redshifts (z>2.5) will be accessible only to powerful X-ray telescopes like ATHENA. We confirm this assumption by means of detailed simulations. The results confirm that such groups will be detected as extended sources by ATHENA, and that the key instrumental parameters are a large efective area (2.1 m^2 at 1 keV) and a good spatial resolution (<10''). <br /> In the third part of this work, the physical differences between galaxy cluster samples selected in different wavebands are investigated. For this research, z>0.8 mid-infrared (SpARCS) and X-ray (XMMLSS) selected cluster samples are compared. We show that the X-ray selected clusters have a more compact distribution of the hot gas and galaxies, compared to the mid-infrared selected clusters. Moreover, we found that a sub-sample of the mid-infrared bright clusters have a lack of X-ray emission, making them consistent with non-virialised systems or filamentary structures projected along the line-of-sight. The final part of the thesis describes a complementary method to probe the baryonic properties of galaxy clusters. The approach consists in constraining the intra-cluster gas pressure model by comparing the predicted thermal Sunyaev-Zel’dovich power spectrum with current Cosmic Microwave Background measurements. We show that a steeper pressure profile in the cluster outskirts, or an evolving gas mass fraction, have mild-to-severe conflicts with the experimental data. It is also shown that the future Sunyaev-Zel’dovich experiments can break the current parameter degeneracies and place simultaneous constraints on the gas pressure profile and its redshift evolution.<br /> The different aspects of the statistical studies based on the galaxy cluster population that we explore in this work, revealed possible sources of uncertainties. These uncertainties deserve careful consideration in order to achieve precision cosmology with future galaxy cluster surveys. Possible routes to improve on this systematics are presented.