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The Forefront of Space Science

Hottest Gas in the Universe Discovered by SUZAKU
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Currently, the most probable theory to explain the origin of the 3 million deg. C gas is that RXJ1347 galaxy cluster recently experienced a violent collision with another galaxy cluster. If two large galaxy clusters collide and merge at an extremely high speed of 4,000km/sec, gas is compressed and heated (Fig. 3). It is guessed that the temperature of the gas, originally around 100 million deg. C, rose to 300 million deg. C with this impact and was observed as extremely high-temperature X-ray radiation.

Figure 3
Figure 3. Computer simulation of collision of galaxy clusters (courtesy of Takuya Akahori, Research Institute of Basic Science, Chungnam National University, Republic of Korea)
From left to right, it is possible to see the evolutionary process where two galaxy clusters collide, gas is compressed and density changes. White and yellow indicate high-density regions compared to other areas.

This is a terrible event. If you were to watch one galaxy cluster from the side of another, it would look like a bullet flying at you. Head-on collision accidents like this, however, are likely to occur sometimes in the universe. The kinetic energy generated at a collision of two galaxy clusters is about 1058 Joule (4 Joule is almost equivalent to 1 Calorie). In the universe created by the Big Bang, these collisions of galaxy clusters are considered the most violent celestial events in terms of energy since the Big Bang.

According to this theory, the life of the 300 million deg. C gas, from birth to disappearance, is about 500 million years. This is an instantaneous event because the history of the universe is more than 10 billion years. I believe we were able to witness a moment in the growth of a galaxy cluster, where two galaxy clusters collided with each other due to their gravity to evolve into a single immense object.

concluding remarks

It was the first time in the world that we observed with great accuracy the extremely high-temperature gas hidden in a galaxy cluster. One key to our success is SUZAKUís capability, which allows us to measure high-energy X-ray with high-sensitivity compared to past observations. We will continue our research of galaxy clusters utilizing the power of SUZAKU. We may discover similar hot area in other galaxy clusters. In addition, the proposed ASTRO-H satellite, a follow-on mission of SUZAKU, will be able to observe precisely the moment of high-speed motion of hot gas triggered by the collision of galaxy clusters. We hope to elucidate the unexpectedly violent evolutionary process of the universe.

Naomi OTA

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