Summary

Using the ESO Very Large Telescope (VLT), a team of astronomers from The
Netherlands, Germany, France and the USA [1] 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.

PR Photo 11a/02: Sky field with the distant cluster of galaxies.

PR Photo 11b/02: Spectra of some of the galaxies in the cluster.

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 [1] 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 distance of about 13.5 billion light years from the Earth.

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 [2]. 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), cf. PR Photo 11a/02. An earlier
example of this observational technique is described in ESO PR 13/99.

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, cf. PR
Photo 11b/02.

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 (10^15) 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 – another, less distant one was
recently described in ESO PR 03/02.

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.

Next steps

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!”

More information

The results described in this Press Release are about to appear in print in
the research journal Astrophysical Journal (“The Most Distant Structure of
Galaxies Known: a Protocluster at z = 4.1” by B.P. Venemans and co-authors),
cf. astro-ph/0203249.

Notes

[1]: The team is led by George Miley (Leiden University, The Netherlands)
and the first author of the resulting research paper is Bram Venemans, a
graduate student of Miley’s. Other members are Jaron Kurk and Huub
Roettgering (also Leiden University), Laura Pentericci (MPIA, Heidelberg,
Germany), Wil van Breugel (University of California, USA), Chris Carilli (US
National Radio Astronomy Observatory, Charlottesville, USA), Carlos De
Breuck (Institut d’Astrophysique, Paris, France) Holland Ford and Tim
Heckman (Johns Hopkins University, Baltimore, USA) and Pat McCarthy
(Carnegie Institute, Pasadena, USA).

[2]: The measured redshift of TN J1338-1942 is z = 4.1. In astronomy, the
redshift denotes the fraction by which the lines in the spectrum of an
object are shifted towards longer wavelengths. The observed redshift of a
remote galaxy provides an estimate of its distance. The distances indicated
in the present text are based on an age of the Universe of 15 billion years.
At the indicated redshift, the Lyman-alpha line of atomic hydrogen (rest
wavelength 121.6 nm) is observed at 620 nm, i.e. in the red spectral region.

Contact

George Miley

Leiden University Observatory

The Netherlands

Tel.: +31-715275849

email: miley@strw.leidenuniv.nl

Technical information about the photos

PR Photo 11a/02 is reproduced from FORS2-exposures, obtained on March 25 and
26, 2001, using a narrow-band optical filter (peak at 619.5 nm with
transmission 80%, FWHM 6.0 nm). The total exposure time was 33300 sec (9 hrs
15 min). The field-of-view of the final image is 6.4 x 6.2 arcmin^2,
corresponding to about 3 Mpc on each side. The frames were obtained in
photometric conditions, and the image quality in the combined frame is 0.65
arcsec. The galaxy spectra shown in PR Photo 11b/02 were obtained by FORS2
in the MXU-mode on May 20, 21 and 22, 2001. Exposures of 31500 sec and 35100
sec, respectively, were made through two masks under photometric conditions,
with seeing 1.0 arcsec and slit sizes of 1 arcsec. The 600RI grism was used;
it has peak efficiency 87%, resolution R = 1011 at 663.0 nm and spectral
dispersion of 0.132 nm/pixel, corresponding to 290 km/s at z = 4.1.