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The Forefront of Space Science

Dynamics in a Galaxy Cluster Explored by SUZAKU
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Challenging the mysteries of the universe's evolution and dark matter

There are at least two significant implications in measuring the velocity of galaxy cluster's plasmas.

First, we can obtain evidence of collision and merger, and explore directly structure formation of the universe. It becomes possible to reproduce structural formation of the universe and model its evolutionary process in computers (actual evolution of the universe takes a long time - literally at a cosmological scale). Meanwhile, we are only able to observe still images of individual celestial bodies. If we could add the movement of celestial bodies (i.e., moving images) to the still images, we can understand more precisely the evolutionary process. As mentioned above, galaxy clusters are evolving: galaxies and plasmas emitting X-ray form galaxy clusters of a variety of scales. Until now, the motion of individual galaxies was measured by the Doppler Effect. The number of galaxies observed, however, was about 100 at most, thus it was difficult to determine which galaxy belonged to which group. Moreover, it is hard to judge if a cluster of galaxies moves together with a cluster of plasmas. In conclusion, it is important to measure dynamics of plasmas and catch 3D "moving images" of a whole galaxy cluster including all the players (i.e., galaxies and plasmas).

Secondly, by measuring plasma dynamics, we can estimate closely the total mass and distribution of dark matter governing motion. Planets, including the earth, orbit around the Sun at different respective speeds. This is because the centrifugal force on planets balances the gravity of the Sun. We believe that, similarly, various forces must balance the gravity produced by the dark matter in galaxy clusters. Until now, the total mass of the dark matter has been estimated on the assumption that thermal pressure of plasmas balances the gravity (thermal pressure = gravity), ignoring the plasma motion. If plasmas move at high speed, however, this assumption cannot be true. For example, if plasmas rotate, the equation becomes "thermal pressure + centrifugal force = gravity." This means that more dark matter is required than previously believed. Our measurement is made for the celestial body in special state, i.e. "merging galaxy cluster." Accordingly, some researchers previously thought the possibility that the presumption "thermal pressure = gravity" cannot be true in such a state. Therefore, our next theme is whether if the plasma movement we discovered exist in "ordinary galaxy clusters"? We need to answer the question to accurately measure the distribution of dark matter.

Finding the "dark components" in the universe with ASTRO-H

There are more than 10,000 galaxy clusters already discovered in the universe. It is believed that they are in various stages of the evolution, so some clusters must be in dynamical state. ASTRO-H has been developed and manufactured as the sixth X-ray satellite of Japan following SUZAKU by the international project of Japan, the U.S. and Europe. ASTRO-H will carry a new type of X-ray detector (X-ray calorimeter) with an energy resolution 20 times higher than that of SUZAKU. With the new instrument incorporating the most advanced technology, we will observe in detail aspects of plasma motion in many galaxy clusters that we were unable to see in the past. By exploring the growth process of large structures in the universe, we will attempt to solve the mystery of the dark matter behind that growth.

There is a possibility that the dark matter will be identified directly and its real identity will be revealed in the near future. Even if the dark matter is identified, however, we think that the mystery of "dark energy" will remain for some time. The "dark energy" is a physical law ruling the entire universe, but its identity is completely unknown. The first step to elucidate dark energy is to measure the distribution and evolution of the dark matter accurately at cosmological scales. To do so, it is crucial to measure galaxy cluster's plasma motion as discussed above. We may be surprised at galaxy clusters' motion beyond our imagination. Why don't you join us to develop new instruments to explore the real identity of the "darkness of the universe"?

Takayuki Tamura

• Masaaki Nagai, Master Thesis (Graduate School, Osaka University, 2008)
• Tamura et al., 2011, PASJ, 63, 1009
• Ustream, ISAS Flash Report, Merger of Galaxy Clusters (2011/11/24)

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