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From: Dan Dubrick
To: All
Date: 2003-06-02 02:06:00
Subject: 5\28 Pt 2 ESO - Extremely Distant Galaxy and FLAMES data

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Part  2 of 3

           Information from the European Southern Observatory

ESO Press Release 12/03

28 May 2003                                                [ESO Logo]

For immediate release

The bright near-infrared night sky

ESO PR Photo 13a/03
Caption: PR Photo 13a/03 shows a spectrum of emission by the
terrestrial atmosphere. In the spectral region above 700 nm, this
emission is dominated by strong lines from the OH molecule. By
observing in "windows" of low OH emission, such as those around 820
or 920 nm, the "noise" caused by the OH-emission is strongly reduced
and it is possible to detect fainter celestial objects.

There is another problem, however, for this kind of work. The search
for faint Lyman-alpha emission from distant galaxies is complicated
by the fact that the terrestrial atmosphere - through which all
ground-based telescopes must look - also emits light. This is
particularly so in the red and near-infrared part of the spectrum
where hundreds of discrete emission lines originate from the hydroxyl
molecule (the OH radical) that is present in the upper terrestrial
atmosphere at an altitude of about 80 km (see PR Photo 13a/03).

This strong emission which the astronomers refer to as the "sky
background" is responsible for the faintness limit at which celestial
objects can be detected with ground-based telescopes at near-infrared
wavelengths. However, there are fortunately spectral intervals of
"low OH-background" where these emission lines are much fainter, thus
allowing a fainter detection limit from ground observations. Two such
"dark-sky windows" are evident in PR Photo 13a/03 near wavelengths of
820 and 920 nm.

Considering these aspects, a promising way to search efficiently for
the most distant galaxies is therefore to observe at wavelengths near
920 nm by means of a narrow-band optical filter. Adapting the
spectral width of this filter to about 10 nm allows the detection of
as much light from the celestial objects as possible when emitted in
a spectral line matching the filter, while minimizing the adverse
influence of the sky emission.

In other words, with a maximum of light collected from the distant
objects and a minimum of disturbing light from the terrestrial
atmosphere, the chances for detecting those distant objects are
optimal. The astronomers talk about "maximizing the contrast" of
objects showing emission lines at this wavelength.

The CFHT Search Programme

ESO PR Photo 13b/03

Caption: PR Photo 13b/03 displays the image of a particular object
(at the center), as seen at various wavelengths (colours) on
CCD-frames obtained through different optical filters with the CFH12K
camera at the CFHT. The object is only visible in the NB920 frame in
which emission at the near-infrared wavelength 920 nm is registered
(upper left). It is not seen in any of the others (Blue [450 nm],
Visual [550 nm], Red [650 nm], I [800 nm]), nor in a combination of
these (the "sum" of BVRI, the so-called "detection" image, here
labeled as "Det"; it is used to detect closer objects from their
optical colours for spectroscopic follow-up observations). The
indicated object was later shown to be an extremely distant galaxy
and has been designated z6VDF J022803-041618.  Each of the six photos
covers 20 x 20 arcsec2; North is up, East is right.

Based on the above considerations, an international team of
astronomers [2] installed a narrow-band optical filter centered at
the near-infrared wavelength 920 nm on the CFH12K instrument at the
Canada-France-Hawaii telescope on Mauna Kea (Hawaii, USA) to search
for extremely distant galaxies. The CFH12K is a wide-field camera
used at the prime focus of the CFHT, providing a field-of-view of
approx. 30 x 40 arcmin2, somewhat larger than the full moon [5].

By comparing images of the same sky field taken through different
filters, the astronomers were able to identify objects which appear
comparatively "bright" in the NB920 image and "faint"
(or are even
not visible) in the corresponding images obtained through the other
filters. A striking example is shown in PR Photo 13b/03 - the object
at the center is well visible in the 920nm image, but not at all in
the other images.

The most probable explanation for an object with such an unusual
colour is that it is a very distant galaxy for which the observed
wavelength of the strong Lyman-alpha emission line is close to 920
nm, due to the redshift.  Any light emitted by the galaxy at
wavelengths shorter than Lyman-alpha is strongly absorbed by
intervening interstellar and intergalactic hydrogen gas; this is the
reason that the object is not visible in all the other filters.

The VLT spectrum

ESO PR Photo 13c/03]                ESO PR Photo 13d/03]

Captions: PR Photo 13c/03 shows a spectroscopic image (between the
horizontal arrows) of the very distant galaxy z6VDF J022803-041618 at
the center of PR Photo 13b/03, obtained with the multi-mode FORS2
instrument at the 8.2-m VLT YEPUN telescope at the ESO Paranal
Observatory. The horizontal axis shows the dispersed light, with
wavelengths increasing from left to right. In this spectral image,
the bright emission lines from OH molecules in the terrestrial
atmosphere, cf. PR Photo 13a/03, have been subtracted, but they still
leave residual "imprints", visible as strong and "noisy" vertical
bars. The "window" at wavelength 920 nm is clearly visible on the
right side of the image; in this region, there is much less "noise"
from the OH-lines. The dark spot at the bottom left of the image is
the Lyman-alpha line of the object. The adjacent "continuum" emission
from the object, although very faint, is clearly visible on the
long-wavelength side (to the right) of the Lyman-alpha line. There is
no such continuum emission detected on the short-wavelength side (to
the left) of the Lyman alpha line. Together with the observed
asymmetry of the line, this is a clear spectral fingerprint of the
redshifted Lyman-alpha emission line from a distant galaxy. PR Photo
13d/03 shows a tracing of the spectrum of this galaxy, as extracted
from the image in PR Photo 13c/03. The strong emission line at
wavelength 872 nm is the redshifted Lyman-alpha spectral line from
the galaxy; it is shown in more detail in the insert panel.

In order to learn the true nature of this object, it is necessary to
perform a spectroscopic follow-up, by observing its spectrum. This
was accomplished with the FORS 2 multi-mode instrument at the 8.2-m
VLT YEPUN telescope at the ESO Paranal Observatory. This facility
provides a perfect combination of moderate spectral resolution and
high sensitivity in the red for this kind of very demanding
observation. The resulting (faint) spectrum is shown in PR Photo

 - Continued -

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