An international team of astronomers 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. 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.
Formation and evolution of galaxies
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 .
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
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.
Colours and distance
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  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
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."
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.
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