An international team of astronomers from Germany and Japan has discovered the most distant cluster of galaxies known so far -- 9.6 billion light years away. The X-ray and infrared observations showed that the cluster hosts predominantly old, massive galaxies, demonstrating that the galaxies formed when the universe was still very young. These and similar observations therefore provide new information not only about early galaxy evolution but also about history of the universe as a whole.
Clusters of galaxies are the largest building blocks in the universe. Our galaxy, the Milky Way, is part of the Virgo cluster, comprising some 1000-2000 galaxies. By observing galaxies and clusters that are very distant from Earth, astronomers can look back in time, as their light was sent out a long time ago and took millions or billions of years to reach the astronomers' telescopes.
An international team of astronomers from the Max Planck Institute for Extraterrestrial Physics, the University of Tokyo and the Kyoto University has now discovered the most distant cluster of galaxies observed so far. X-ray observations in the Subaru XMM Deep Field helped to identify the candidates, and infrared observations using the Subaru telescope provided the distance information. A particularity of this discovery consists in using infrared wavelengths, invisible to the naked eye. This is dictated by the expansion of the universe, which forces distant galaxies to have large velocities, shifting their light away from visible to infrared wavelengths. The Multi-Object Infrared Camera and Spectrometer (MOIRCS) at the Subaru telescope works at near-infrared wavelengths, where the galaxies are most luminous.
"The MOIRCS instrument has an extremely powerful capability of measuring distances to galaxies. This is what made our challenging observation possible," says Masayuki Tanaka from the University of Tokyo. "Although we confirmed only several massive galaxies at that distance, there is convincing evidence that the cluster is a real, gravitationally bound cluster."
That the individual galaxies are indeed held together by gravity is confirmed by observations in a very different wavelength regime: The matter between the galaxies in clusters is heated to extreme temperatures and emits light at much shorter wavelengths than visible to the human eye. The team therefore used the XMM-Newton space observatory to look for this radiation in X-rays.
"Despite the difficulties in collecting X-ray photons with a small effective telescope size similar to the size of a backyard telescope, we detected a clear signature of hot gas in the cluster," explains Alexis Finoguenov from the Max Planck Institute for Extraterrestrial Physics.
The combination of these different observations in (to the eye) invisible wavelengths therefore led to the pioneering discovery of the galaxy cluster at a distance of 9.6 billion light years -- some 400 million light years further into the past than the previously most distant cluster known.
An analysis of the data collected about the individual galaxies shows that the cluster contains already an abundance of evolved, massive galaxies that formed some two billion years earlier. As the dynamical processes for galaxy aging are slow, presence of these galaxies requires the cluster assembly through merger of massive galaxy groups, each nourishing its dominant galaxy. The cluster is therefore an ideal laboratory for studying the evolution of galaxies, when the universe was only about a third of its present age.
As distant galaxy clusters are also important tracers of the large scale structure and primordial density fluctuations in the universe, similar observations in the future will lead to important information for cosmologists. The results obtained so far demonstrate that current near infrared facilities are capable of providing a detailed analysis of distant galaxy populations and that the combination with X-ray data is a powerful new tool. The team therefore is continuing the search for more distant clusters.
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