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New X-ray Astronomy Satellite ASTRO-H Striving to Solve the Mysteries of the Universe

New ways of looking at the universe

Q. What is the major achievement of your research to date?

The galaxy cluster RXJ1347-1145 at a distance of 5 billion light years from Earth. In the area marked with a circle, gas was detected at over 200 million degrees Kelvin. Left: image from the Subaru Telescope (courtesy: NAOJ). Right: image from the Chandra X-ray Observatory. (courtesy: NASA)

It was previously thought that gas in a galaxy cluster couldn’t get much hotter than 100 million degrees Kelvin. However, by combining X-ray and millimeter wave observations, we found gas that is much hotter than that – over 200 million degrees Kelvin. How did it get so hot? We think it started out at around 100 million degrees Kelvin, but got much hotter when galaxy clusters collided at the incredible speed of 4,000 kilometers per second and drastically compressed the gas. We probably captured the process in which clusters collided and joined together to make up a bigger cluster. Even so, an inferred speed of 4,000 kilometers per second greatly exceeds traditional theoretical expectations, and I was very surprised.

Q. What kind of research do you expect to perform with ASTRO-H?

ASTRO-H can perform the highest-quality spectrum measurement of all current X-ray astronomy satellites, so I’m expecting to make precise studies of the energy distribution of gas in a galaxy cluster, and to directly measure gas motions. I previously mentioned that to heat gas to over 200 million degrees Kelvin, a collision velocity of 4,000 kilometers per second would be necessary, and ASTRO-H may be able to turn such an estimation into a “measurement.” This work will deepen our understanding of the formation of galaxy clusters and the heating process of gas.
Launching a new satellite means that we will have a “new eye” – we will get to see what we could not see before. I am looking forward to seeing what this new eye will teach us about the universe.

ASCA featured cutting-edge observation methods

Q. What is the most impressive result from Japanese X-ray astronomy satellites to date?

X-ray image of the “dark galaxy cluster” AX J2019+1127, captured by the X-ray satellite ROSAT. In this area, Japan’s X-ray astronomy satellite ASCA discovered high-temperature gas at approximately 100 million degrees Kelvin. The brighter sections indicate stronger X-ray emissions. (courtesy: Hattori et al. (1997) Nature 388, 146)
X-ray image of the “dark galaxy cluster” AX J2019+1127, captured by the X-ray satellite ROSAT. In this area, Japan’s X-ray astronomy satellite ASCA discovered high-temperature gas at approximately 100 million degrees Kelvin. The brighter sections indicate stronger X-ray emissions. (courtesy: Hattori et al. (1997) Nature 388, 146)

AX J2019+1127 observed by the Chandra X-ray Observatory. These lights turned out to be not a galaxy cluster but several small celestial objects twinkling individually. (courtesy: Chartas et al. (2001) The Astrophysical Journal 550, L163)
AX J2019+1127 observed by the Chandra X-ray Observatory. These lights turned out to be not a galaxy cluster but several small celestial objects twinkling individually. (courtesy: Chartas et al. (2001) The Astrophysical Journal 550, L163)

I started working in this field when I was a graduate student in the mid- 1990s, which happened to overlap with the period when a Japanese X-ray astronomy satellite, ASCA, was active. So I got ASCA’s results in real time, and these results really stand out in my memory.
ASCA had many achievements, so it is hard for me to pick just one, but specifically its report on the discovery of a “dark galaxy cluster” left a large impression on me. At the time, there was a mysterious area where the gravitational lensing effect was detected but no celestial object causing it could be seen. Through ASCA’s X-ray observations, hot gas with a temperature of about 100 million degrees Kelvin was detected and recognized as a galaxy cluster. If this was a real galaxy cluster, it would be among the most distant ones ever discovered with X-rays. It was bright only in X-rays and appeared to lack galaxies that normally would be observed in optical wavelengths. This object became a hot topic and was called a “dark galaxy cluster.”
Unfortunately, after a few years, this interpretation turned out to be incorrect. When Chandra X-ray Observatory imaged the same area, it turned out this wasn’t a galaxy cluster but just a group of several small celestial objects that were shining individually.
Despite this unfortunate result, the method – studying distant objects through a combination of the gravitational lensing effect and X-ray observations – was very refreshing, and has been applied widely ever since. We have learned various deep lessons through a series of events related to the dark galaxy cluster, such as the importance of developing new methods and the need to make the most of the strengths of instruments while fixing their weaknesses at the same time.

Astronomy as the ultimate exploration

Q. What do you think is the charm of X-ray astronomy?

I think one of the charms of X-ray astronomy is the relative ease of direct comparisons between basic physics and observational data. What I’m trying to do is substantive research; in other words, verification through a combination of theories and observations. The temperature and speed of gas in a galaxy cluster is closely related to the process of its formation, so to some extent, it is possible to test our theories against the observational data.

Q. Can you tell us your future goals?

I think astronomy in general is the ultimate kind of exploration. In other words, the universe is the ultimate object of our curiosity. My major goal is to make connections between the evolution of the universe and the formation of various celestial objects. To accomplish this goal, I initially want to deepen my understanding of galaxy clusters to help exploration of galaxies, stars and eventually life inside them.

Tetsu Kitayama, Ph.D.

Associate Professor at the Department of Physics, Faculty of Science, Toho University
Dr. Kitayama graduated from the Department of Physics, Faculty of Science at the University of Tokyo in 1993, and received his Ph.D. at the Graduate School of Science at the University of Tokyo in 1998. Subsequently he was a fellow of the Japan Society for the Promotion of Science. Starting in 2001 he was a lecturer, and from 2004 an associate professor, at the Department of Physics, Faculty of Science, Toho University. His specialty is observational cosmology – the formation and evolution of galaxies and galaxy clusters.

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