Intergalactic Medium In The Universe

Our understanding of the universe have been much increased recent years. Many astronomical observations have reached distant past. The galaxies survey of SDSS has observed QSOs upto z (?) 6.5. From HI absorption in the spectra of QSOs we know IGM were highly ionized at z < 6. They are becoming less ionized at z > 6. WMAP satellite has obseved the scatter optical depth of electron, which indicates IGM has been ionized at z (?) 11. We used a simple model to describe the process of structure formation, and calculated reionization history considering the thermal feedback. We found they are consistent with observations.We can also observe HeII Lyα absorption in the spectra of QSOs thanks to Far-UV satellite FUSE. The column density ratio of HeII to HI can be used to constrain UV radiation background. Observation has found the ratio are very scattered at scales about 1Mpc. It is difficult to explain them using the distribution of the slope of QSOs' radiation, because QSOs are too sparse at that time. We studied the effects of thermal broadening and velocity field using hydrodynamic simulations. We also included the effect of observational noises. We found the high ratio events can be explained well, while the simulated low ratio events are too little compared with observation. They may be from some processes not included in our calculation.We also studied the homogenity of UV radiation background at z (?) 6. We found the rapid increase of optical depth scatter does not imply UV radiation background is also fluctuated. We also studied Xray background from gases heated during structure formation. We found shock heating is enough to explain the entropy of clusters. And we also calculated Xray background from diffuse IGM and found they are negligible and consistent with observations. We also studied the statistical behaviors of the simulated IGM at z=0 and found they are similar to turbulence.