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Galaxy cluster in 'early' universe is unexpectedly mature

10 July 2011

Astronomers using the Very Large Telescope in Chile have
proved that a galaxy cluster with ancient galaxies and stars existed a mere three billion years after the big bang.

Astronomers expect that clusters of galaxies grow through time and hence that massive clusters would be rare or none existent in the early Universe. "We have measured the distance to the most distant mature cluster of galaxies ever found," says the lead author of the study, Raphael Gobat (CEA, Paris). "The surprising thing is that when we look closely at this galaxy cluster it doesn't look young — many of the galaxies have settled down and don't resemble the star-forming galaxies expected to be seen in the early Universe."

The international team of astronomers measured the distances to some of the blobs in a curious patch of very faint red objects first observed with theSpitzer space telescope. This grouping, named [1], had all the hallmarks of being a very remote cluster of galaxies [2]. The results showed that we are indeed seeing a galaxy cluster as it was when the Universe was about three billion years old — less than one quarter of its supposed current age [3].

Once the team knew the distance to this very rare object they looked carefully at the component galaxies using both the Hubble Space Telescope and ground-based telescopes, including the the Very Large Telescope. They found evidence suggesting that most of the galaxies in the cluster were not forming stars, but were composed of stars that were already at least one billion years old. This makes the cluster a mature object, similar in mass to the Virgo Cluster, the nearest rich galaxy cluster to the Milky Way.

A composite image showing the most remote mature cluster of galaxies yet found
This image is a composite of very long exposures taken with ESO’s Very Large Telescope in Chile and the NAOJ’s Subaru telescope on Hawaii. Most of the visible objects are very faint and distant galaxies. The clump of faint red objects to the right of center is the most remote mature cluster of galaxies yet found. Credit: ESO/R. Gobat et al.

Further evidence that this is a mature cluster comes from observations of X-rays coming from CL J1449+0856 made with ESA's XMM-Newton space observatory. The cluster is giving off X-rays that must be coming from a very hot cloud of tenuous gas filling the space between the galaxies and concentrated towards the centre of the cluster. This is another sign of a mature galaxy cluster, held firmly together by its own gravity, as very young clusters have not had time to trap hot gas in this way.

As Gobat concludes: "These new results support the idea that mature clusters existed when the Universe was less than one quarter of its current age. Such clusters are expected to be very rare according to current theory, and we have been very lucky to spot one. But if further observations find many more then this may mean that our understanding of the early Universe needs to be revised."


1. The strange name refers to the object's position in the sky.

2. The galaxies appear red in the picture partly because they are thought to be mainly composed of cool, red stars. In addition the expansion of the Universe since the light left these remote systems has increased the wavelength of the light further so that it is mostly seen as infrared radiation when it gets to Earth.

3. The astronomers measured the distance to the cluster by splitting the light up into its component colours in a spectrograph. They then compared this spectrum with one of a similar object in the nearby Universe. This allowed them to measure the redshift of the remote galaxies -- how much the Universe has expanded since the light left the galaxies. The redshift was found to be 2.07, which means that the cluster is seen at a distance that equates to three billion years after the Big Bang.

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