VLT Discovers a Group of Early Inhabitants and Find Signs of Many
More [1]

Using the ESO Very Large Telescope (VLT), two astronomers from
Germany and the UK [2] have discovered some of the most distant
galaxies ever seen. They are located about 12,600 million
light-years away.

It has taken the light now recorded by the VLT about nine-tenths
of the age of the Universe to traverse this huge distance. We
therefore observe those galaxies as they were at a time when
the Universe was very young, less than about 10% of its present
age. At this time, the Universe was emerging from a long period
known as the "Dark Ages", entering the luminous "Cosmic
Renaissance" epoch.

Unlike previous studies which resulted in the discovery of a
few, widely dispersed galaxies at this early epoch, the present
study found at least six remote citizens within a small sky
area, less than five per cent the size of the full moon! This
allowed understanding the evolution of these galaxies and how
they affect the state of the Universe in its youth.

In particular, the astronomers conclude on the basis of their
unique data that there were considerably fewer luminous
galaxies in the Universe at this early stage than 500 million
years later.

There must therefore be many less luminous galaxies in the
region of space that they studied, too faint to be detected
in this study. It must be those still unidentified galaxies
that emit the majority of the energetic photons needed to
ionise the hydrogen in the Universe at that particularly

From the Big Bang to the Cosmic Renaissance

Nowadays, the Universe is pervaded by energetic ultraviolet
radiation, produced by quasars and hot stars. The short-
wavelength photons liberate electrons from the hydrogen atoms
that make up the diffuse intergalactic medium and the latter
is therefore almost completely ionised. There was, however, an
early epoch in the history of the Universe when this was not

The Universe emanated from a hot and extremely dense initial
state, the so-called Big Bang. Astronomers now believe that
it took place about 13,700 million years ago.

During the first few minutes, enormous quantities of protons,
neutrons and electrons were produced. The Universe was so hot
that protons and electrons were floating freely: the Universe
was fully ionised.

After some 100,000 years, the Universe had cooled down to a
few thousand degrees and the nuclei and electrons now combined
to form atoms. Cosmologists refer to this moment as the
"recombination epoch". The microwave background radiation we
now observe from all directions depicts the state of great
uniformity in the Universe at that distant epoch.

However, this was also the time when the Universe plunged into
darkness. On one side, the relic radiation from the primordial
fireball had been stretched by the cosmic expansion towards
longer wavelengths and was therefore no more able to ionise
the hydrogen. On the contrary, it was trapped by the hydrogen
atoms just formed. On the other side, no stars nor quasars had
yet been formed which could illuminate the vast space. This
sombre era is therefore quite reasonably dubbed the "Dark Ages".
Observations have not yet been able to penetrate into this
remote age — our knowledge is still rudimentary and is all
based on theoretical calculations.

A few hundred million years later, or at least so astronomers
believe, some very first massive objects had formed out of
the huge clouds of gas that had moved together. The first
generation of stars and, somewhat later, the first galaxies
and quasars, produced intensive ultraviolet radiation. That
radiation could not travel very far, however, as it would be
immediately absorbed by the hydrogen atoms which were again
ionised in this process.

The intergalactic gas thus again became ionised in steadily
growing spheres around the ionising sources. At some moment,
these spheres had become so big that they overlapped
completely: the fog over the Universe had lifted !

This was the end of the Dark Ages and, with a term again
taken over from human history, is sometimes referred as the
"Cosmic Renaissance". Describing the most significant
feature of this period, astronomers also call it the "epoch
of reionisation".

Finding the Most Distant Galaxies with the VLT

To cast some light on the state of the Universe at the end of
the Dark Ages, it is necessary to discover and study extremely
distant (i.e. high-redshift [2]) galaxies. Various
observational methods may be used — for instance, distant
galaxies have been found by means of narrow-band imaging
(e.g., ESO PR 12/03), by use of images that have been
gravitationally enhanced by massive clusters, and also

Matthew Lehnert from the MPE in Garching, Germany, and Malcolm
Bremer from the University of Bristol, UK, used a special
technique that takes advantage of the change of the observed
colours of a distant galaxy that is caused by absorption in
the intervening intergalactic medium. Galaxies at redshifts
of 4.8 to 5.8 [2] can be found by looking for galaxies which
appear comparatively bright in red optical light and which
are faint or undetected in the green light. Such "breaks" in
the light distribution of individual galaxies provide strong
evidence that the galaxy might be located at high redshift
and that its light started on its long journey towards us,
only some 1,000 million years after the Big Bang.

For this, they first used the FORS2 multi-mode instrument on
the 8.2-m VLT YEPUN telescope to take extremely "deep"
pictures through three optical filters (green, red and
very-red) of a small area of sky (40 square arcmin, or approx.
5 percent the size of the full moon). These images revealed
about 20 galaxies with large breaks between the green and red
filters, suggesting that they were located at high redshift.
Spectra of these galaxies were then obtained with the same
instrument, in order to measure their true redshifts.

"The key to the success of these observations was the use of
the great new red-enhanced detector available on FORS2", says
Malcolm Bremer.

The spectra indicated that six galaxies are located at
distances corresponding to redshifts between 4.8 and 5.8;
other galaxies were closer. Surprisingly, and to the delight
of the astronomers, one emission line was seen in another
faint galaxy that was observed by chance (it happened to be
located in one of the FORS2 slitlets) that may possibly be
located even further away, at a redshift of 6.6. If this
would be confirmed by subsequent more detailed observations,
that galaxy would be a contender for the gold medal as the
most distant one known!

The Earliest Known Galaxies

The spectra revealed that these galaxies are actively forming
stars and are probably no older than 100 million years,
perhaps even younger. However, their numbers and observed
brightness suggest that luminous galaxies at these redshifts
are fewer and less luminous than similarly selected galaxies
nearer to us.

"Our findings show that the combined ultraviolet light from
the discovered galaxies is insufficient to fully ionise the
surrounding gas", explains Malcom Bremer. "This leads us to
the conclusion that there must be many more smaller and less
luminous galaxies in the region of space that we studied, too
faint to be detected in this way. It must be these still
unseen galaxies that emit the majority of the energetic
photons necessary to ionise the hydrogen in the Universe."

"The next step will be to use the VLT to find more and fainter
galaxies at even higher redshifts", adds Matthew Lehnert.
"With a larger sample of such distant objects, we can then
obtain insight into their nature and the variation of their
density in the sky."

A British Premiere

The observations presented here are among the first major
discoveries by British scientists since the UK became a
member of ESO in July 2002. Richard Wade from the Particle
Physics and Astronomy Research Council (PPARC), which funds
the UK subscription to ESO, is very pleased: "In joining the
European Southern Observatory, UK astronomers have been
granted access to world-leading facilities, such as the VLT.
These exciting new results, of which I am sure there will
be many more to come, illustrate how UK astronomers are
contributing with cutting-edge discoveries."

More information

The results described in this Press Release are about to
appear in the research journal Astrophysical Journal
("Luminous Lyman Break Galaxies at z>5 and the Source of
Reionization" by M. D. Lehnert and M. Bremer). It is
available electronically as astro-ph/0212431.


[1]: This is a coordinated ESO/PPARC Press Release.

[2]: This work was carried out by Malcolm Bremer (University
of Bristol, The United Kingdom) and Matthew Lehnert (Max-
Planck-Institut fur Extraterrestrische Physik, Garching,

[3]: The measured redshifts of the galaxies in the Bremer Deep
Field are z = 4.8-5.8, with one unexpected (and surprising)
redshift of 6.6. 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 13.7 billion years. At the indicated
redshift, the Lyman-alpha line of atomic hydrogen (rest
wavelength 121.6 nm) is observed at 680 to 920 nm, i.e. in
the red spectral region.