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12\11 ESO - Deepest Infrared View of the Universe
Part 1 of 2

        Information from the European Southern Observatory
 ESO Press Release 23/02
 11 December 2002                                         [ESO Logo]
 For immediate release
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Deepest Infrared View of the Universe

VLT Images Progenitors of Today's Large Galaxies
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Summary
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An international team of astronomers [2] has made the deepest-ever
near-infrared Ks-band image of the sky, using the ISAAC multi-mode
instrument on the 8.2-m VLT ANTU telescope.

For this, the VLT was pointed for more than 100 hours under optimal
observing conditions at the Hubble Deep Field South (HDF-S) and 
obtained images in three near-infrared filters. The resulting images 
reveal extremely distant galaxies, which appear at infrared 
wavelengths, but are barely detected in the deepest optical images 
acquired with the Hubble Space Telescope (HST).

Astronomer Marijn Franx from the University of Leiden and leader of 
the team concludes: "These results demonstrate that very deep 
observations in the near-infrared are essential to obtain a proper 
census of the earliest phases of the universe. The new VLT images 
have opened a new research domain which has not been observationally 
accessible before".

The HDF-S is a tiny field on the sky in the southern constellation 
Tucana (The Toucan) - only about 1% of the area of the full moon. The 
NASA/ESA Hubble Space Telescope (HST) observed it with a total 
exposure time of about 1 week, yielding the deepest optical images 
ever taken of the sky, similar to those made earlier on the Hubble 
Deep Field North (HDF-N).

The VLT infrared images of the same field were obtained in the course 
of a major research project, the Faint InfraRed Extragalactic Survey 
(FIRES).  They were made at wavelengths up to 2.3 micron where the 
HST is not competitive.

Ivo Labbe, another team member from the University of Leiden, is 
certain:  "Without the unique capabilities of the VLT and ISAAC we 
would never have been able to observe these very remote galaxies. In 
fact, the image in the Ks-band is the deepest which has ever been 
made at that wavelength".

The optical light emitted by the distant galaxies has been redshifted 
to the near-infrared spectral region [3]. Indeed, some of the 
galaxies found in the new images are so remote that - due to the 
finite speed of light - they are observed as they were when the 
Universe was still extremely young, less than 2 billion years old.

From these observations, two interesting conclusions have been drawn 
so far. One is that although the newly identified galaxies do not 
appear to form stars very actively they probably account for about 
half the mass of normal matter present at this epoch. This is in 
sharp contrast to the galaxies at this early time found during 
optical surveys - they are very blue because of young and hot stars.

Another is that galaxies existed already at that epoch which are 
clearly rather large, and some show spiral structure similar to that 
seen in very nearby galaxies.

This new important insight is having profound impact on the current
attempts to understand the formation and evolution of galaxies.

PR Photo 28a/02: Composite colour image of the sky field observed by 
HST and VLT.  PR Photo 28b/02: The ISAAC Ks-band image, the deepest 
of its kind ever obtained.  PR Photo 28c/02: Images of very red, very 
distant compact galaxies in different wavebands.  PR Photo 28d/02: 
Images of very distant extended galaxies in different wavebands.
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Formation and evolution of galaxies
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How did galaxies form in the early Universe? How did they evolve and 
when did the first stars form in those systems?

These are some of the key questions in present-day astronomy. Thanks 
to powerful ground- and space-based telescopes, astronomers are now 
able to actively pursue studies in this direction. Recent front-line 
observational results are helping them to gain new insights into these 
fundamental issues.

Light emitted by distant galaxies travels a long time before we 
observe it with our telescopes. In this way, astronomers can look back 
in time and directly study galaxies as they were when the universe was 
still very young.  However, this is technically difficult, as the 
galaxies are extremely faint.  Another complication is that, due to 
the expansion of the universe, their light is shifted towards longer 
wavelengths [3].

In order to study those early galaxies in some detail, astronomers 
thus need to use the largest ground-based telescopes, collecting their 
faint light during very long integrations. And they must work in the 
infrared region of the spectrum which is not visible to the human eye.

The Hubble Deep Field South (HDF-S) was selected to be studied in 
great detail with the Hubble Space Telescope (HST) and other powerful 
telescopes.  The HST images of this field represent a total exposure 
time of 140 hours.  Many ground-based telescopes have obtained 
additional photos and spectra, in particular telescopes at the 
European Southern Observatory in Chile.

The ISAAC observations
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ESO PR Photo 28a/02                ESO PR Photo 28b/02
[Preview - JPEG: 400 x 479 pix     [Preview - JPEG: 400 x 477 pix
 - 71k                               - 86k 
[Normal - JPEG: 800 x 958 pix      [Normal - JPEG: 800 x 954 pix
 - 304k]                            - 408k]
[Full-Res - JPEG: 2301 x 2756 pix  [Full-Res - JPEG: 2302 x 2746 pix
 - 3.7M]                             - 3.8M]

 Caption: PR Photo 28a/02 shows a three-colour composite image of the 
 small sky field observed with the ISAAC multi-mode instrument at VLT 
 ANTU during the FIRES project. The central field observed by the HST 
 WFPC2-camera is outlined in white. The photo is a combination of one 
 HST exposure (in the I-filter at wavelength 0.814 micron; here 
 rendered as blue) and two ISAAC exposures (Js; 1.24 micron; green - 
 Ks; 2.16 micron; red). A striking variety of colours is evident, 
 reflecting the different types and distances of the galaxies in this 
 field. PR Photo 28b/02 is a reproduction of the ISAAC Ks-image with a 
 total exposure time of 35.6 hours during optimal observing 
 conditions, and with extraordinary image sharpness, 0.46 arcsec. It 
 is the deepest image ever obtained in this waveband. The field 
 measures 2.5 x 2.5 arcmin^2; North is up and East is left.

The sky field in the direction of HDF-S observed in the present study 
(the Faint InfraRed Extragalactic Survey (FIRES)), measures 2.5 x 2.5 
arcmin2. It is slightly larger than the field covered by the WFPC2 
camera on the HST, but still 100 times smaller than the full moon.

Whenever the field was visible from Paranal and the atmospheric 
conditions were optimal, ESO astronomers pointed the 8.2-m VLT ANTU 
telescope in the direction of this field, taking near-infrared images 
with the ISAAC multi-mode instrument. The data were transmitted by 
Internet to the astronomers of the team in Europe, who then combined 
them to construct some of the deepest infrared astronomical images 
ever taken from the ground.

(continued)

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