Deepest Infrared View of the Universe

Composite colour image of the sky field observed by HST and VLT (ESO PR Photo 28a/02).

VLT Images Progenitors of Today’s Large Galaxies [1]

An international team of astronomers [2] has made the deepest-evernear-infrared Ks-band image of the sky, using the ISAAC multi-modeinstrument on the 8.2-m VLT ANTU telescope.

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

Astronomer Marijn Franx from the University of Leiden and leader ofthe team concludes: “These results demonstrate that very deepobservations in the near-infrared are essential to obtain a propercensus of the earliest phases of the universe. The new VLT images haveopened a new research domain which has not been observationallyaccessible before”.

The HDF-S is a tiny field on the sky in the southern constellationTucana (The Toucan) – only about 1% of the area of the full moon. TheNASA/ESA Hubble Space Telescope (HST) observed it with a totalexposure time of about 1 week, yielding the deepest optical imagesever taken of the sky, similar to those made earlier on the HubbleDeep Field North (HDF-N).

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

Ivo Labbe, another team member from the University of Leiden, iscertain: “Without the unique capabilities of the VLT and ISAAC wewould never have been able to observe these very remote galaxies. Infact, the image in the Ks-band is the deepest which has ever been madeat that wavelength”.

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

From these observations, two interesting conclusions have beendrawn so far. One is that although the newly identified galaxies donot appear to form stars very actively they probably account for abouthalf the mass of normal matter present at this epoch. This is insharp contrast to the galaxies at this early time found during opticalsurveys – they are very blue because of young and hot stars.

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

This new important insight is having profound impact on the currentattempts 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 whendid the first stars form in those systems?

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

Light emitted by distant galaxies travels a long time before we observe itwith our telescopes. In this way, astronomers can look back in time anddirectly 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, theirlight is shifted towards longer wavelengths [3].

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

The Hubble Deep Field South (HDF-S) was selected to be studied in greatdetail 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, inparticular 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 (theFaint InfraRed Extragalactic Survey (FIRES)), measures 2.5 x 2.5 arcmin2. Itis 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 conditionswere optimal, ESO astronomers pointed the 8.2-m VLT ANTU telescope in thedirection of this field, taking near-infrared images with the ISAACmulti-mode instrument. The data were transmitted by Internet to theastronomers of the team in Europe, who then combined them to construct someof 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 threeinfrared bands (Js, H, Ks), allowing a 3-dimensional view of a small regionof the Universe. This is because, by comparing the brightness of thegalaxies in these colours with that in optical light, as measured by theHST, it is possible to estimate their redshifts [3] and thus how long agothe light we now see has been emitted.

For the reddest of the galaxies the answer is that we are seeing them asthey 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 aretherefore all the more important in the context of formation and evolutionof galaxies.

One is that a few of them are clearly rather large and show spiral structuresimilar to that seen in very nearby galaxies, cf. PR Photo 28d/02. It is notobvious that current theoretical models can easily account for such galaxieshaving 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 opticalwavelengths, most of the ’infrared-selected’ galaxies show relatively littlevisible star-forming activity. They appear in fact to have already formedmost of their stars and in quantities sufficient to account for at leasthalf the total luminous mass of the Universe at that time. Given the time toreach this state they must clearly have formed even earlier in the life ofthe Universe and are thus probably amongst the “oldest” galaxies now known.

Rather than being randomly distributed in space, these red galaxies are alsofound to prefer company, i.e., they tend to cluster close to each other. Ingeneral terms this can be taken as support for the latest theoretical modelsin which galaxies, which consist of “normal” matter, form in thehighest-density regions of the much more pervasive “dark” matter. Althoughthe latter accounts for most of the mass of the universe, its origin so faris completely unknown.

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

More information

The information presented in this Press Release is based on a researcharticle (“Ultradeep Near-Infrared ISAAC Observations of the Hubble DeepField South: Observations, Reduction, Multicolor Catalog, and PhotometricRedshifts” by Ivo Labbe et al.) that will soon appear in the researchjournal “Astronomical Journal” (cf. astro-ph/0212236). A shorter accountwill appear in the December 2002 issue of ESO’s house journal “TheMessenger”. Information, including photos and reduced data, is alsoavailable at the website of the FIRES project.

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