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Published online by Cambridge University Press: 25 May 2016
X-ray emission from clusters of galaxies is the most useful tool in studying mass distribution and chemical compositions in these enormously large systems. The hot intracluster medium (ICM) has been heated up to kT = 3–10 keV during the gravitational collapse, and X-ray luminosities indicate that the gas is more massive than the total galaxy mass contained in clusters by factors of 3–5. This makes ICM the dominant form of baryons in the universe. In many clusters observations indicate that ICM is in a hydrostatic equilibrium within a potential governed by the dark matter, and the cooling time is longer than the Hubble time except for the bright centers. The ICM, therefore, enables us a close look at the structure of gravitational potential. At the same time, heavy-element abundances in the ICM and their distribution are used to estimate past supernova activities and metal injection mechanism in cluster space.