Galaxy cluster literally means a cluster of many galaxies and is the largest celestial body in the universe. Looking at a galaxy cluster in visible light, it looks simply as if many galaxies had been pulled together due to their gravity. When observing in X-ray, however, a clear picture emerges showing the cluster wholly surrounded by hot gas at a temperature of several tens of thousands to 100 million deg. C (Fig. 1). Even the total gravity of galaxies forming the cluster is far too little to confine such hot gas in a restricted space. Therefore, it is thought that a substantial amount of as-yet-unidentified dark matter is hidden within the cluster. Nonetheless, galaxy clusters are not uncommon, because more than 10,000 clusters have been discovered until now. The Milky Way galaxy that includes our own solar system is believed to be part of a galaxy cluster called the Virgo Cluster. Thus, we are actually residing in a galaxy cluster.
When were these giant objects such as galaxy clusters born and how have they grown? By observing a far-distant universe, we can know the picture of our universe in the past. Hence, we thought that we could investigate the evolution of galaxy clusters by close observation of far-distant galaxy clusters.
Discovery of the universeís hottest gas in a galaxy cluster
We focused on a distant object called the RXJ1347 galaxy cluster. This galaxy cluster is located about 5 billion light years from the earth. It extends to about 5 million light years in total and is known as one of the brightest galaxy clusters in the whole sky, especially, when observed in the X-ray wavelength. Although it is located far away, the cluster is very bright. Therefore, the cluster is a suitable one for us to investigate how gases and dark matter are distributed within it.
Before the launch of the X-ray astronomical satellite SUZAKU, observations of the cluster were conducted by Europeís ROSAT, Japanís ASCA, and the USís Chandra satellites etc. Late in the 1990s, it was reported that gas cooler than that of the surrounding area existed in the center of the cluster. In general, there is a tendency for gas density to increase as it approaches the center of a galaxy cluster. In such regions, radiative cooling is prone to occur and temperature declines over time. Judging from the observation results and this theory, we speculated that RXJ1347 galaxy cluster had fully evolved over a long period of time since its birth and was a gentle object. As our X-ray and radiowave observations advanced, however, the following new facts were discovered.
Using the SUZAKU satellite launched in 2005, we observed the RXJ1347 galaxy cluster in detail twice on June 30, 2006 and July 15, 2006. As a result of the detailed analysis of the observed data, we successfully obtained decisive and surprising evidence that extremely hot gas reaching up to 300 million deg. C exists in the cluster.