Original Source: European Southern Observatory

April 09, 2002

An extremly distant cluster of galaxies at redshift 4.1

FORS2 spectrograph at the VLT 8.2m KUEYEN telescope Credit: ESO

Using the ESO Very Large Telescope (VLT), a team of astronomers from The Netherlands, Germany, France and the USA have discovered the most distant group of galaxies ever seen, about 13.5 billion light-years away. It has taken the light now recorded by the VLT about nine-tenths of the age of the Universe to cover the huge distance. We therefore observe those galaxies as they were at a time when the Universe was only about 10% of its present age.

The astronomers conclude that this group of early galaxies will develop into a rich cluster of galaxies, such as those seen in the nearby Universe. The newly discovered structure provides the best opportunity so far for studying when and how galaxies began to form clusters after the initial Big Bang, one of the greatest puzzles in modern cosmology.

Radio Galaxies as cosmic signposts

A most intriguing question in modern astronomy is how the first groupings or "clusters" of galaxies emerged from the gas produced in the Big Bang.

Some theoretical models predict that densely populated galaxy clusters ("rich clusters" in current astronomical terminology) are built up through a step-wise process. Clumps develop in the primeval gas, and stars condense out of these clumps to form small galaxies. Then these small galaxies merge together to form larger units.

The peculiar class of "radio galaxies" is particularly important for investigating such scenarios. They are called so because their radio emission - a result of violent processes believed to be related to massive black holes located at the centres of these galaxies - is stronger by 5 - 10 orders of magnitude than that of our own Milky Way galaxy. In fact, this radio emission is often so intense that the galaxies can be spotted at extremely large distances, and thus at the remote epoch when the Universe was very young, just a small fraction of its present age.

The radio galaxies are amongst the most massive objects in the early Universe and there has long been circumstantial evidence that they are located at the heart of young clusters of galaxies, still in the process of formation. In this sense, they act as signposts of early cosmic "meeting points".

Radio galaxies are therefore potential beacons for pinpointing regions of the Universe in which large galaxies and clusters of galaxies are being formed.

VLT observations of the environment of radio galaxy TN J1338-1942

Following up this conjecture, the Leiden astronomers and their colleagues in the USA and Germany proposed a large observing programme with the ESO VLT at Paranal (Chile) to search for groupings of galaxies in the vicinity of distant radio galaxies that might be the ancestors of rich clusters.

For this, they first used the FORS2 multi-mode instrument on the 8.2-m VLT KUEYEN telescope to take very "deep" pictures of sky regions around several radio galaxies, each field measuring about one-fifth of the diameter of the full moon. The most distant of these was an object called TN J1338-1942, a radio galaxy at a redshift of 4.1, i.e. at a distance of about 13.5 billion light years from the Earth. We see it as it was when the Universe was just 1.5 billion years old. A stream of hydrogen gas stretches from it to the northwest, over a distance of about 300,000 light-years.

Starfield with most distant galaxy cluster Blue circles: galaxies - box: radio galaxy Credit: ESO

To search for galaxies at the same distance as the radio galaxy, the pictures were optimised in sensitivity for the sharp colour emitted by glowing hydrogen gas at the distance of the radio galaxy. Images were taken through two red filters, one that is "tuned" to light produced by the hydrogen gas (the redshifted Lyman-alpha line) and the other that is dominated by light from stars (the R-band).

These images revealed 28 galaxies that are likely to be at the distance of the radio galaxy. More detailed information was obtained for 23 of these with the FORS2 instrument in the spectrographic mode, now confirming 20 of them to be indeed located at the same distance as the radio galaxy.

Earliest known group of galaxies

The spectra also showed that the galaxies are moving around with speeds of a few hundred kilometers per second. The observed structure of galaxies is more than 10 million light-years across and its existence means that galaxies must have begun to form groups already at this early epoch, i.e. still within the first 10% of the history of the Universe.

From the excess number of detected galaxies and the observed volume of the structure, its combined mass can be estimated. The derived number is 1000 million million (10¹⁵) times the mass of the Sun - this is comparable with the masses of nearby rich clusters of galaxies. For the present structure to evolve into a nearby rich cluster, it must contract in volume by an order of magnitude in about one billion years.

This newly discovered group of galaxies is the most remote discovered so far and hence the earliest known at this moment.

The VLT observations also establish a crucial link between the ancestors of rich galaxy clusters and the bright galaxies whose active nuclei produce the bright radio emission. Based on the 4 radio galaxies surveyed by the VLT so far, the team concludes that every forming cluster may house a bright galaxy that is or has been a powerful radio source. The radio sources are believed to be powered by massive black holes located deep within their nuclei.

The next step in the present project will be to use the VLT to establish the boundaries of the proto-cluster. Also, the colours and shapes of galaxies in the structure will be studied intensively by the Advanced Camera for Surveys (ACS), recently fitted to the Hubble Space Telescope (HST).

George Miley, also a member of the ACS Science Team, is enthusiastic: "We have now scheduled this particular target for one of the deepest observations ever to be made with the HST. Our project is an example of the great possibilities now opening to astronomers by combining the complementary strengths of the wonderful new ground- and space-based observational facilities!"

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see also:

• Very Remote Galaxy - Surplus of Intergalactic Material May Be Young Supercluster (ESO March 11, 2002)


• astro-ph/0203249 - Preprint of ApJ paper "The Most Distant Structure of Galaxies Known: a Protocluster at z = 4.1" by B.P. Venemans et al.