A Cross-Correlation Study In The Epoch Of Reionization, Dark Matter In Galaxy Clusters, And Modified Gravity

There are three parts in this thesis. Part one is on the detection of the reionization epoch, part two focuses on dark matter in galaxy clusters, and part three presents our study of the modified gravity theory as an alternative of dark matter and dark energy.Part one of the thesis is a contribution to the theoretical studies of the 21CMA project, one of NAOC's key project which aims at probing the epoch of reionization. The reionization epoch proceeds a period from the formation of a large amount of the first stars at z～20 to z～6 when the universe was totally ionized. This is a crucial stage of structure formation which is related to the galaxy formation and the heating, kinetic and chemical processes of the IGM. Meanwhile, the Cosmic Infrared Background (CIRB) and the Soft X-ray Background (SXRB) contain unclear components, they may come from the early universe. The UV band photons from PopIII stars and the hard X-ray photons from QSOs can be redshilted to the present infrared and soft X-ray band, respectively. These signals provide us an additional probe of the epoch of reionization. Because of the strong foregrounds of all these signals, we present a cross-correlation study. The reason is that cross-correlations can highlight the correlated signals and eliminate irrelevant foregrounds. In this thesis, we study the CIRB from PopIII stars and SXRB from QSOs. We develop a shell model to describe the 21cm signals and find that PopIII stars can provide higher 21cm signals than QSOs. At last, we predict various cross power spectra analytically, and study their detectability combined with observations (21CMA/LOFAR, ROSAT and AKARI). We find that although these cross-correlation signals have distinct features, they are difficult to be detected so far, due to the high noise of the soft X-ray and infrared backgrounds given by ROSAT and AKARI.The second part is the study of dark matter in galaxy clusters. Our research focuses on: (1) Dark matter ring in the CL0024+17 (Jee et al. 2007). Because of the same spatial distributions of dark matter and cluster galaxies, we study the galaxy distribution to probe the ringlike structure in the dark matter distribution. We find that, at about 3σlevel, the ringlike structure seen in the dark matter measurement is not observed in the projected 2D galaxy distribution. (2) Offset between dark matter and baryonic matter center in clusters. The X-ray and lensing study of the bullet cluster show a clear offset between dark matter and baryonic matter center. This observation gave us the direct evidence that dark matter exist on cluster scales. We point out such an offset is common in the universe from a sample of clusters, and explain the long-standing mass discrepancy between strong gravitational lensing and X-ray problem in galaxy clusters.The third part is the research about the modified gravity theory (MOND) and its covariant formalism (TeVeS). We study the gravitational lensing of covariant MOND, we develop an analytic model for non-spherical lenses in covariant MOND, and study such a model with CASTLES survey. We use this model to obtain a reasonable MOND fit to most of the double-imaged systems. However, we also find those non-isolated double-imaged systems can not be fitted well. And most quadruple-imaged systems for which no reasonable MOND fit can be obtained with our models. We argue that, this is mostly due to the intrinsic limitation of the analytic models, we indeed showed that our models were unable to produce a large Einstein ring and a large shear at the same time.