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From: Dan Dubrick
To: All
Date: 2003-06-02 02:06:00
Subject: 5\28 Pt 3 ESO - Extremely Distant Galaxy and FLAMES data

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Part  3 of 3

PR Photo 13d/03 shows a tracing of the final ("cleaned") spectrum of
the object after extraction from the image shown in PR Photo
13c/03. One broad emission line is clearly detected (to the left of
the center; enlarged in the insert). It is asymmetric, being
depressed on its blue (left) side.  This, combined with the fact that
no continuum light is detected to the left of the line, is a clear
spectral signature of the Lyman-alpha line: photons "bluer" than
Lyman-alpha are heavily absorbed by the gas present in the galaxy
itself, and in the intergalactic medium along the line-of-sight
between the Earth and the object.

The spectroscopic observations therefore allowed the astronomers to
identify unambiguously this line as Lyman-alpha, and therefore to
confirm the great distance (high redshift) of this particular
object. The measured redshift is 6.17, making this object one of the
most distant galaxies ever detected. It received the designation
"z6VDF J022803-041618" - the first part of this somewhat unwieldy
name refers to the survey and the second indicates the position of
this galaxy in the sky.

Starlight in the early Universe

However, these observations did not come without surprise! The
astronomers had hoped (and expected) to detect the Lyman-alpha line
from the object at the center of the 920 nm spectral window. However,
while the Lyman-alpha line was found, it was positioned at a somewhat
shorter wavelength.

Thus, it was not the Lyman-alpha emission that caused this galaxy to
be "bright" in the narrow-band (NB920) image, but "continuum"
emission at wavelengths longer than that of Lyman-alpha. This
radiation is very faintly visible as a horizontal, diffuse line in PR
Photo 13c/03. 

One consequence is that the measured redshift of 6.17 is lower than
the originally predicted redshift of about 6.5. Another is that z6VDF
J022803-041618 was detected by light from its massive stars (the
"continuum") and not by emission from hydrogen gas (the Lyman-alpha
line).

This interesting conclusion is of particular interest as it shows
that it is in principle possible to detect galaxies at this enormous
distance without having to rely on the Lyman-alpha emission line,
which may not always be present in the spectra of the distant
galaxies. This will provide the astronomers with a more complete
picture of the galaxy population in the early Universe.

Moreover, observing more and more of these distant galaxies will
help to better understand the ionization state of the Universe at
this age: the ultraviolet light emitted by these galaxies should not
reach us in a "neutral" Universe, i.e., before re-ionization
occurred. The hunt for more such galaxies is now on to clarify how
the transition from the Dark Ages happened!

More information

The research described in this press release is presented in a
research Letter to the Editor, soon to appear in the European
research journal Astronomy & Astrophysics ("Discovery of a z=6.17
galaxy from CFHT and VLT observations" by Jean-Gabriel Cuby et al.).
It is available on the web as astro-ph/0303646.

Notes

[1] This press release is issued jointly by ESO and the
Canada-France-Hawaii Telescope Corporation (CFHT).

[2] The team consists of Jean-Gabriel Cuby (ESO), Olivier Le Fevre,
Baptiste Meneux (Laboratoire d'Astrophysique de Marseille, France),
Henry McCracken (Osservatorio Astronomico di Bologna, Italy),
Jean-Charles Cuillandre, Eugene Magnier (Canada-France-Hawaii
Telescope Corporation, Hawaii, USA).

[3]: In astronomy, the "redshift" denotes the fraction by which the
lines in the spectrum of an object are shifted towards longer
wavelengths. Since the redshift of a cosmological object increases
with distance, the observed redshift of a remote galaxy also provides
an estimate of its distance.  Moreover, assuming a particular
cosmological model, the redshift provides a measure of the look-back
time and hence at which epoch the light was emitted that is now
observed. The distances indicated in the present text are based on an
age of the Universe of 13.7 billion years, conforming with recent
estimates based on the observed fluctuations in the microwave
background radiation. For a redshift of 6.17, the Lyman-alpha line of
atomic hydrogen (rest wavelength 121.6 nm) is observed at wavelength
871.9 nm, i.e. in the near-infrared spectral region. The
corresponding look-back time is 93.3% of the age of the Universe and
we observe an object with this redshift as it was when the age of the
Universe was 6.7% (i.e., about 900 million years) of the current age.

[4] Quasars are particularly active galaxies, the centres of which
emit prodigious amounts of energy and energetic particles. It is
believed that they harbour a massive black hole at their centre and
that the energy is produced when surrounding matter falls into this
black hole. This type of object was first discovered in 1963 by the
Dutch-American astronomer Maarten Schmidt at the Palomar Observatory
(California, USA) and the name refers to their "star-like" appearance
on the images obtained at that time.

[5]: It has now been replaced by MEGACAM, a 1 square degree camera
providing CFHT with the largest wide-field imager on a 4-m class
telescope in the world.

Contact

Jean-Gabriel Cuby
ESO
Paranal Observatory, Chile
Phone: +56 55 43 5317
email: jcuby{at}eso.org

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