A 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 must have formed earlier than 9.6 billion years ago, ie. 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 1,000 to 2,000 galaxies.
By observing galaxies and clusters that are very distant from Earth, astronomers can look back in time, as the galaxies’ light was emitted a long time ago and took millions or billions of light-years to reach the astronomers’ telescopes.
Invisible to the naked eye
Astronomers had to use infrared wavelengths, invisible to the naked eye, because the expansion of the universe — which forces distant galaxies to have large velocities — shifts their light from visible to infrared wavelengths.
The Multi-Object Infrared Camera and Spectrometer (MOIRCS) at the Subaru Telescope detects near-infrared wavelengths, at which 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 band. The gas between the galaxies in clusters is heated to extreme temperatures and emits light at much shorter wavelengths. 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 … we detected a clear signature of hot gas in the cluster,” explains Alexis Finoguenov from the Max Planck Institute for Extraterrestrial Physics.
Record smashed by 400 million light-years
The combination of these different observations 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 processes for galaxy aging are slow, the presence of these galaxies means the cluster must have come about through the merger of massive galaxy groups, each nourishing its dominant galaxy.
The team is continuing the search for more distant clusters.
Adapted from information issued by MPE /NAOJ / Subaru.