Abstract
In the past years significant efforts have been focused on describing the evolution and structure of the Universe. One of the big open questions in cosmology is "Where is the baryonic mass"? We know from various surveys that the number of detected baryons in the Universe at present days is much smaller than predicted by the standard big-bang nucleosynthesis model and measured by the detailed observation of the Lyman-alpha forest at redshift z=2. Hydrodynamical simulations indicate that a large fraction of the baryons today is expected to be in a "warm-hot" (10 5-107 K) filamentary gas, distributed in the intergalactic medium. This gas, if exists, should be highly ionized and observable only in the soft x-ray and UV bands. The first part of my project is dedicated to studying the evolution of the baryonic mass in the intergalactic medium using the output of hydrodynamic simulations. In the second part I use the same simulations to predict the x-ray flux from the Warm-Hot Intergalactic Medium (WHIM), its distribution in space and its spectral properties. The primary motivation of this investigation is the construction and launch of a dedicated mission to detect and study the properties of the intergalactic medium and the missing baryons. What I found is that, depending on the model, in the 0.37-0.925 keV energy band the WHIM is between 3% and 20% of the Diffuse X-Ray Background, and comes mostly from filaments between redshift 0.1 and 0.8. The filaments have typical angular size of a few arc minutes, which requires an detector angular resolution of 2' or less. Since the identification of the WHIM is possible only from emission lines, we need a detector with energy resolution in the order of 4 eV or less. Another critical issue is the detector background which needs to be less than 10-3 counts s -1 eV-1 in order to detect the weak signal from the WHIM.