Studies of two distant galaxy clusters
using a combination of the largest radio, optical and x-ray
telescopes on the ground and in space have independently
found that galaxies formed relatively early in the history
of the Universe. The two galaxy clusters studied are
respectively the most distant proto-cluster ever found and
the most massive known galaxy cluster for its epoch.

Looking back in time nearly 9 billion years, an
international team of astronomers found mature galaxies in
a young Universe. The galaxies are members of a cluster of
galaxies that existed when the Universe was only 5 billion
years old, or about 35 percent of its present age. This is
compelling evidence that galaxies must have started forming
just after the Big Bang and is bolstered by observations
made by the same team of astronomers when they peered even
farther back in time. The team found embryonic galaxies a
mere 1.5 billion years after the birth of the cosmos, or 10
percent of the Universe’s present age. They reside in a still
developing cluster, the most distant proto-cluster ever found.

The Advanced Camera for Surveys (ACS) aboard the NASA/ESA
Hubble Space Telescope was used to make the observations of
the massive cluster, RDCS1252.9-2927, and the proto-
cluster, TNJ1338-1942. Observations by NASA’s Chandra X-ray
Observatory yielded the mass and heavy element content of
RDCS1252.9-2927, the most massive known cluster for that
epoch. These observations are part of a coordinated effort
by the ACS science team to track the formation and
evolution of clusters of galaxies over a broad span of
cosmic time. The ACS was specially built for such studies
of very distant objects.

These findings support the theory that galaxies formed
relatively early in the history of the cosmos. The
existence of such massive clusters in the early Universe
agrees with a cosmological model wherein clusters form by
the merger of many sub-clusters in a Universe dominated by
cold dark matter. The precise nature of cold dark matter,
however, is still not known.

The first Hubble study estimated that the galaxies in
RCDS1252 formed the bulk of their stars more than 11
billion years ago (redshifts greater than 3). The results
were published in the October 20, 2003 issue of the
Astrophysical Journal. The paper’s lead author is John
Blakeslee of the Johns Hopkins University in Baltimore,

The second Hubble study uncovered, for the first time, a
proto-cluster of “infant galaxies” that existed more than
12 billion years ago (redshift 4.1). These galaxies are so
young that astronomers can still see a flurry of stars
forming within them. The galaxies are grouped around one
large galaxy. These results will be published in the
January 1, 2004 issue of Nature. The paper’s lead author is
George Miley of Leiden Observatory in the Netherlands.

“Until recently people didn’t think that clusters existed
when the Universe was only about 5 billion years old,”
Blakeslee explained.

“Even if there were such clusters,” Miley added, “until
recently astronomers thought it was almost impossible to
find clusters that existed 8 billion years ago. In fact, no
one really knew when clustering began. Now we can witness

Both studies led the astronomers to conclude that these
systems are the progenitors of the galaxy clusters seen
today. “The cluster RDCS1252 looks like a present-day
cluster,” said Marc Postman of the Space Telescope Science
Institute in Baltimore, USA, and co-author of both research
papers. “In fact, if you were to put it next to a present-
day cluster you wouldn’t know which is which.”

A Tale of Two Clusters
How can galaxies grow so fast after the Big Bang? “It is a
case of the rich getting richer,” Blakeslee said. “These
clusters grew quickly because they are located in very
dense regions, so there is enough material to build up the
member galaxies very fast.”

This idea is bolstered by X-ray observations of the massive
cluster RDCS1252. Chandra and the European Space Agency’s
XMM-Newton provided astronomers with the most accurate
measurements to date of the properties of an enormous cloud
of hot gas that pervades the massive cluster. This 70
million-degree Celsius gas is a reservoir of most of the
heavy elements in the cluster, and an accurate tracer of
its total mass. A paper by Piero Rosati of the European
Southern Observatory (ESO) and colleagues that presents the
X-ray observations of RDCS1252 will be published in January
2004 in the Astronomical Journal.

“Chandra’s sharp vision resolved the shape of the hot gas
halo and showed that RDCS1252 is very mature for its age,”
said Rosati, who discovered the cluster with the ROSAT X-
ray telescope.

RDCS1252 may contain many thousands of galaxies. Most of
those galaxies, however, are too faint to detect, although
the powerful “eyes” of the ACS pinpointed several hundred
of them. Observations using ESO’s Very Large Telescope
(VLT) provided a precise measurement of the distance to the
cluster. The ACS enabled the researchers to determine the
shapes and the colours of the 100 galaxies accurately,
providing information on the ages of the stars residing in
them. The ACS team estimated that most of the stars in the
cluster were already formed by the time the Universe was
about 2 billion years old. In addition X-ray observations
showed that 5 billion years after the Big Bang the
surrounding hot gas had been enriched with heavy elements
from these stars and swept away from the galaxies.

If most of the galaxies in RDCS1252 have reached maturity
and are settling into a quiet adulthood, the galaxies
forming in the distant proto-cluster are in their
energetic, unruly youth.

The proto-cluster TN J1338 contains a massive embryonic
galaxy surrounded by smaller developing galaxies, which
look like dots in the Hubble image. The dominant galaxy is
producing spectacular radio-emitting jets, fueled by a
supermassive black hole deep within the galaxy’s nucleus.
Interaction between these jets and the gas can stimulate a
torrent of star birth.

The discovery of the energetic radio galaxy by radio
telescopes prompted astronomers to hunt for the smaller
galaxies that make up the bulk of the cluster.

“Massive clusters are the cities of the Universe, and the
radio galaxies within them are the smokestacks we can use
for finding them when they are just beginning to form,”
Miley said.

The two findings underscore the power of combining
observations from many different telescopes to provide
views of the distant Universe over a range of wavelengths.
Hubble’s advanced camera provided critical information on
the structure of both distant galaxy clusters. Chandra’s
and XMM-Newton’s X-ray vision furnished the essential
measurements of the primordial gas in which the galaxies in
RDCS1252 are embedded, and accurate estimates of the total
mass contained within that cluster. Large ground-based
telescopes, like the VLT, provided precise measurements of
the distance of both clusters as well as the chemical
composition of the galaxies in them.

The ACS team is conducting further observations of distant
clusters to solidify our understanding of how these young
clusters and their galaxies evolve into the shape of things
seen today. Their planned observations include using near-
infrared observations to analyse the star-formation rates
in some of their clusters, including RDCS1252, in order to
measure the cosmic history of star formation in these
massive structures. The team is also searching the regions
around several ultra-distant radio galaxies for additional
examples of proto-clusters. The team’s ultimate scientific
goal is to establish a complete picture of cluster
evolution beginning with their formation at the earliest
epochs and detailing their evolution up to the present

Notes for editors

For broadcasters, animations of the discovery and general
Hubble Space Telescope background footage are available

For more information, please contact:

George Miley
Leiden Observatory, Leiden, the Netherlands
Tel: +31 71 527 849

Piero Rosati
European Southern Observatory, Munich, Germany
Tel: +49 89 320 06 589

John Blakeslee
Johns Hopkins University, Baltimore, USA
Tel: +1 410 516 8835

Marc Postman
Space Telescope Science Institute, Baltimore, USA
Tel: +1 410 516 8762

Lars Lindberg Christensen
Hubble European Space Agency Information Centre, Munich,
Tel: +49 (0)89 3200 6306
Cellular (24 hr): +49(0)173 3872 621

Donna Weaver
Space Telescope Science Institute, Baltimore, USA
Tel: +1 410 338 4493

Megan Watzke
Chandra X-ray Observatory Center, CfA, Cambridge, USA
Tel: +1 617 496 7998