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Stripping down to the
cosmic skeleton

Posted: November 03, 2009

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A previously unknown assembly of galaxies has been detected 6.7 billion light years away, the first observation of such a prominent galaxy structure in the distant Universe.

“Matter is not distributed uniformly in the Universe,” says Masayuki Tanaka from ESO, who led the new study. “In our cosmic vicinity, stars form in galaxies and galaxies usually form groups and clusters of galaxies. The most widely accepted cosmological theories predict that matter also clumps on a larger scale in the so-called ‘cosmic web’, in which galaxies, embedded in filaments stretching between voids, create a gigantic wispy structure.”

This illustration shows the position of the galaxies and reveals the extent of this gigantic structure. The galaxies located in the newly discovered structure are shown in red. Galaxies that are either in front or behind the structure are shown in green. Image: ESO/L. Calçada/Subaru/National Astronomical Observatory of Japan/M. Tanaka.

The Universe's main-frame can be imagined as these flowing filaments, which span distances of millions of light years. Galaxies assemble around the filaments with immense clusters converging at their intersections where, over time, they accumulate more and more galactic members.

While observed in the near Universe, understanding how massive filamentary structures swirl into existence at greater distances has proved something of a challenge, until now. By combining spectroscopic data from the VIMOS instrument on ESO's Very Large Telescope and FOCAS on the Subaru telescope (operated by the National Astronomical Observatory of Japan), Tanaka and colleagues could study the spiderweb-like structure in great detail.

The galaxies located in the newly discovered structure are shown in red. Galaxies that are either in front or behind the structure are shown in blue. Image: ESO/Subaru/National Astronomical Observatory of Japan/M. Tanaka.

Measuring the distances from Earth of over 150 galaxies allowed the astronomers to obtain a three-dimensional view of the structure, which extends over at least 60 million light years. They identified tens of separate groups of galaxies surrounding the main galaxy cluster, each between ten and a thousand times as massive as our own Milky Way Galaxy, with the mass of the main cluster estimated as at least ten thousand times the mass of the Milky Way. Some of the clumps are feeling the fatal gravitational pull of the cluster, and will eventually fall into it.

“This is the first time that we have observed such a rich and prominent structure in the distant Universe,” says Tanaka. “We can now move from demography to sociology and study how the properties of galaxies depend on their environment, at a time when the Universe was only two thirds of its present age.”

The team have already planned future observations to obtain a definite measure of the structure's size.

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