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

Colours and distance
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A crucial feature of the new observations is that they were made in 
three infrared bands (Js, H, Ks), allowing a 3-dimensional view of a 
small region of the Universe. This is because, by comparing the 
brightness of the galaxies in these colours with that in optical 
light, as measured by the HST, it is possible to estimate their 
redshifts [3] and thus how long ago the light we now see has been 
emitted.

For the reddest of the galaxies the answer is that we are seeing them 
as they were when the Universe was only about 2 billion years old.

The nature of the galaxies
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ESO PR Photo 28c/02             ESO PR Photo 28d/02
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 PR Photos 28c-d/02 display images of some of the galaxies in the 
 Hubble Deep Field South, as they appear in different colours, 
 including the V+I (HST - 0.55 + 0.81 micron; left) visual band, the 
 near-infrared Ks-band (VLT - 2.16 micron; middle), together with 
 optical-to-infrared I,J,K-colour composites (HST+VLT; right). In PR 
 Photo 28c/02, three very red galaxies, all at large distances, are 
 found to be very bright in the infrared. The upper two have compact 
 shapes, whereas the galaxy at the bottom is very large, comparable to 
 the size of the Milky Way galaxy in which we live.  Red galaxies like 
 these that were found in the present survey are a major constituent 
 of the Universe at high redshift. Three other galaxies in PR Photo 
 28d/02 are equally distant but are bluer and their images are also 
 extended. There are indications of star formation in some knots in 
 the rudimentary spiral arms. The large galaxies represent a class 
 never before seen at this large distance and they look surprisingly 
 similar to giant spiral galaxies like our Milky Way galaxy.

Two conclusions drawn so far about the nature of these galaxies are
therefore all the more important in the context of formation and 
evolution of galaxies.

One is that a few of them are clearly rather large and show spiral 
structure similar to that seen in very nearby galaxies, cf. PR Photo 
28d/02. It is not obvious that current theoretical models can easily 
account for such galaxies having evolved to this stage so early in the 
life of the Universe.

Another conclusion is that, in contrast to the galaxies at similar 
redshifts (and hence, at this early epoch) found most commonly in 
surveys at optical wavelengths, most of the 'infrared-selected' 
galaxies show relatively little visible star-forming activity. They 
appear in fact to have already formed most of their stars and in 
quantities sufficient to account for at least half the total luminous 
mass of the Universe at that time. Given the time to reach this state 
they must clearly have formed even earlier in the life of the Universe 
and are thus probably amongst the "oldest" galaxies now known.

Rather than being randomly distributed in space, these red galaxies 
are also found to prefer company, i.e., they tend to cluster close to 
each other. In general terms this can be taken as support for the 
latest theoretical models in which galaxies, which consist of "normal" 
matter, form in the highest-density regions of the much more pervasive 
"dark" matter. Although the latter accounts for most of the mass of 
the universe, its origin so far is completely unknown.

These new observations may, therefore, also add new insight into one 
of the biggest mysteries currently confronting cosmologists. Marijn 
Franx agrees, but also cautions against drawing firm conclusions on 
this aspect too quickly: "We now need similar images of a considerably 
larger region of the sky. We will soon follow-up these first, 
tantalizing results with more observations of other sky fields."

More information
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The information presented in this Press Release is based on a research
article ("Ultradeep Near-Infrared ISAAC Observations of the Hubble 
Deep Field South: Observations, Reduction, Multicolor Catalog, and 
Photometric Redshifts" by Ivo Labbe et al.) that will soon appear in 
the research journal "Astronomical Journal" (cf. astro-ph/0212236). A 
shorter account will appear in the December 2002 issue of ESO's house 
journal "The Messenger". Information, including photos and reduced 
data, is also available at the website of the FIRES project.

Notes

[1]: This press release is issued in coordination between ESO, Leiden
Observatory, the Netherlands Research School for Research in Astronomy
(NOVA) and the Netherlands Foundation for Research (NWO). A 
Dutch-language version is available here.

[2]: The team consists of Ivo Labbe, Marijn Franx, Natascha M. 
Foerster Schreiber, Paul van der Werf, Huub Roettgering, Lottie van 
Starkenburg, Arjen van de Wel and Konrad Kuijken (Leiden Observatory, 
The Netherlands), Gregory Rudnick (Max-Planck-Institut fuer 
Astrophysik, Garching, Germany), Hans-Walter Rix (Max-Planck-Institut 
fuer Astronomie, Heidelberg, Germany), Alan Moorwood and Emanuele 
Daddi (ESO, Garching, Germany) and Pieter G. van Dokkum (California 
Institute of Technology, Pasadena, USA).

[3]: 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.

Contacts

Marijn Franx
Leiden Observatory
The Netherlands
Phone: ++31-71-527-5870
email: franx{at}strw.leidenuniv.nl

Ivo Labbe
Leiden Observatory
The Netherlands
Phone: ++31-71-527-5805
email: ivo{at}strw.leidenuniv.nl

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