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From: Dan Dubrick
To: All
Date: 2003-06-02 02:06:00
Subject: 5\28 Pt 1 ESO - Extremely Distant Galaxy and FLAMES data

 This Echo is READ ONLY !   NO Un-Authorized Messages Please!
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Part  1 of 3

           Information from the European Southern Observatory

ESO Press Release 12/03

28 May 2003                                                [ESO Logo]

For immediate release
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CFHT and VLT Identify Extremely Distant Galaxy

Top Telescopes Peer into the Distant Past [1]

Summary

With improved telescopes and instruments, observations of extremely
remote and faint galaxies have become possible that were until
recently astronomers' dreams.

One such object was found by a team of astronomers [2] with a
wide-field camera installed at the Canada-France-Hawaii telescope at
Mauna Kea (Hawaii, USA) during a search for extremely distant
galaxies. Designated "z6VDF J022803-041618", it was detected because
of its unusual colour, being visible only on images obtained through
a special optical filter isolating light in a narrow near-infrared
band.

A follow-up spectrum of this object with the FORS2 multi-mode
instrument at the ESO Very Large Telescope (VLT) confirmed that it is
a very distant galaxy (the redshift is 6.17 [3]). It is seen as it
was when the Universe was only about 900 million years old.

z6VDF J022803-041618 is one of the most distant galaxies for which
spectra have been obtained so far. Interestingly, it was discovered
because of the light emitted by its massive stars and not, as
originally expected, from emission by hydrogen gas.

PR Photo 13a/03: Emission from the Earth's atmosphere.
PR Photo 13b/03: CHFT images of the very remote galaxy z6VDF
                 J022803-041618.
PR Photo 13c/03: VLT spectrum of very remote galaxy z6VDF
                 J022803-041618.
PR Photo 13d/03: Cleaned tracing of the VLT spectrum.
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A brief history of the early Universe

Most scientists agree that the Universe emanated from a hot and
extremely dense initial state in a Big Bang. The latest observations
indicate that this crucial event took place about 13,700 million
years ago.

During the first few minutes, enormous quantities of hydrogen and
helium nuclei with protons and neutrons were produced. There were
also lots of free electrons and during the following epoch, the
numerous photons were scattered from these and the atomic nuclei. At
this stage, the Universe was completely opaque.

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. The photons were then no longer scattered from these and the
Universe suddenly became transparent. 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.

In the next phase, the primeval atoms - more than 99% of which were
of hydrogen and helium - moved together and began to form huge clouds
from which stars and galaxies later emerged. The first generation of
stars and, somewhat later, the first galaxies and quasars [4],
produced intensive ultraviolet radiation. That radiation did not
travel very far, however, despite the fact that the Universe had
become transparent a long time ago.  This is because the ultraviolet
(short-wavelength) photons would be immediately absorbed by the
hydrogen atoms, "knocking" electrons off those atoms, while
longer-wavelength photons could travel much farther. The
intergalactic gas thus again became ionized in steadily growing
spheres around the ionizing sources.

At some moment, these spheres had become so big that they overlapped
completely; this is referred to as the "epoch of re-ionization".
Until then, the ultraviolet radiation was absorbed by the atoms, but
the Universe now also became transparent to this radiation. Before,
the ultraviolet light from those first stars and galaxies could not
be seen over large distances, but now the Universe suddenly appeared
to be full of bright objects. It is for this reason that the time
interval between the epochs of "recombination" and
"re-ionization" is
referred to as the "Dark Ages".

When was the end of the "Dark Ages"?

The exact epoch of re-ionization is a subject of active debate among
astronomers, but recent results from ground and space observations
indicate that the "Dark Ages" lasted a few hundred million
years. Various research programmes are now underway which attempt to
determine better when these early events happened. For this, it is
necesary to find and study in detail the earliest and hence, most
distant, objects in the Universe - and this is a very demanding
observational endeavour.

Light is dimmed by the square of the distance and the further we look
out in space to observe an object - and therefore the further back in
time we see it - the fainter it appears. At the same time, its dim
light is shifted towards the red region of the spectrum due to the
expansion of the Universe - the larger the distance, the larger the
observed redshift [3].

The Lyman-alpha emission line

With ground-based telescopes, the faintest detection limits are
achieved by observations in the visible part of the spectrum. The
detection of very distant objects therefore requires observations of
ultraviolet spectral signatures which have been redshifted into the
visible region. Normally, the astronomers use for this the redshifted
Lyman-alpha spectral emission line with rest wavelength 121.6 nm; it
corresponds to photons emitted by hydrogen atoms when they change
from an excited state to their fundamental state.

One obvious way of searching for the most distant galaxies is
therefore to search for Lyman-alpha emission at the reddest (longest)
possible wavelengths. The longer the wavelength of the observed
Lyman-alpha line, the larger is the redshift and the distance, and
the earlier is the epoch at which we see the galaxy and the closer we
come towards the moment that marked the end of the "Dark Ages".

CCD-detectors used in astronomical instruments (as well as in
commercial digital cameras) are sensitive to light of wavelengths up
to about 1000 nm (1 micron), i.e., in the very near-infrared spectral
region, beyond the reddest light that can be perceived by the human
eye at about 700-750 nm.

 - Continued -

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