ESO Press Release 10/00
 
Starting in the evening of April 1, 2000, Ghislain Golse and Francisco Castander from the Observatoire Midi-Pyrenees (Toulouse, France) [1] were the first "visiting astronomers" at Paranal to carry out science observations with the second 8.2-m VLT Unit Telescope, KUEYEN.
 
Using the FORS2 multi-mode instrument as a spectrograph, they measured the distances to a number of very remote galaxies, located far out in space behind two clusters of galaxies. Such observations may help to determine the values of cosmological parameters that define the geometry and fate of the Universe.
 
After two nights of observations, the astronomers came away from Paranal with a rich harvest of data and a good feeling. "We are delighted that the telescope performed so well. It is really impressive how far out one can reach with the VLT, compared to the `smaller’ 4-meter telescopes with which we previously observed. It opens a new window towards the distant, early Universe. Now we are eager to start reducing and analysing these data!", Francisco Castander said.
 
Measuring the Geometry of the Universe with Multiple Images in Cluster Lenses
 
The present programme is typical of the fundamental cosmological studies that are now being undertaken with the ESO Very Large Telescope (VLT).
 
Clusters of galaxies are very massive objects. Their gravitational fields intensify ("magnify") and distort the images of galaxies behind them. The magnification factor for the faint background galaxy population seen within a few arcminutes of the centre of a massive cluster at intermediate distance (redshift z ~ 0.2 – 0.4, i.e., corresponding to a look-back time of approx. 2 – 4 billion years) is typically larger than 2, and occasionally much larger. The clusters thus function as gravitational lenses. They may be regarded as "natural telescopes" that help us to see fainter objects further out into space than would otherwise be possible with our own telescopes.
 
In a few cases, the images of the objects behind the clusters are split into several components. Knowing the distance to the objects for which we see multiple images and the distribution of matter in the cluster that produce the lensing effect allows to determine the geometry of the universe in the corresponding direction, independently of its rate of expansion.
 
For a given cluster lens, a minimum of three such multiple-imaged objects with measured distances and positions is in principle sufficient to determine the geometry of the universe in that direction, as expressed by the values of two of the main cosmological parameters, the density (Omega) and the cosmological constant (Lambda). Detailed observations of these cosmic mirages thus have a direct implication for our understanding of the universe in which we live.
 
A study of the clusters of galaxies Abell 1689 and MS 1008
 
The first visiting astronomers to KUEYEN used FORS2 to measure the distances to some of the background objects that are being multiple-lensed by the cluster of galaxies Abell 1689. This cluster was first discovered by American astronomer George Abell some thirty years ago when he studied photographic plates obtained at the Palomar Observatory.
 
Since then, this cluster has been further observed and deep images taken by the Hubble Space Telescope (HST) have revealed at least five multiple-lensed objects in this direction.
 
However, because of the faintness of these images, it has so far not been possible to measure the distances to those objects. This has only become possible now, with the advent of new and powerful astronomical instruments like the FORS2 spectrograph at KUEYEN.
 
At the beginning of the night — before Abell 1689 was high enough in the sky to be observable — the astronomers also observed another cluster lens, MS 1008. This cluster was discovered with the Einstein X-ray satellite and has been studied in great detail by means of images in different colours by the VLT ANTU telescope during the Science Verification phase.
 
Spectra of distant lensed objects
 
  ESO PR Photo 10a/00
 
  Caption: Multi-colour image of the field in the galaxy cluster MS 1008,   with a 24.5-mag lensed quasar (arrow) observed at redshift z = 4.0 during   the present study. This image was obtained by the VLT/ANTU telescope   during its Science Verification phase. The photo is based on a composite   of four images with exposure times and seeing conditions of 82 min and   0.72 arcsec (B band), 90 min and 0.65 arcsec (V band), 90 min and 0.64   arcsec (R band) and 67 min and 0.55 arcsec (I band), respectively. The   field is 1.8 x 1.6 arcmin2; North is up and East is left.
 
  ESO PR Photo 10b/00
 
  Caption: The spectrum obtained with FORS2 at KUEYEN of a quasar at redshift   z = 4.0, lensed by the massive cluster of galaxies MS 1008. The redshifted   Lyman-alpha line from hydrogen (rest wavelength 1216 A in the far-   ultraviolet part of the spectrum) is clearly seen in emission at 6025 A as   a high peak in the red spectral region. Another emission line, from four   times ionized nitrogen (rest wavelength 1240 A), is seen in the right wing   of the Lyman-alpha line. The spectrum was obtained after two hours of   exposure through a 1.0 arcsec slit in good atmospheric conditions (seeing:   0.6 arcsec).
 
With the comparatively large field-of-view of FORS2 at VLT KUEYEN, the Toulouse team obtained spectra of very faint objects, not only in the cluster core region where the multiple-lensed background galaxies are found, but also in the outer regions of the cluster where the images of objects are not split into several images, but only magnified.
 
One of the faint objects (Photo 10a/00) turned out to be a very distant quasar with a redshift of about z = 4.0, as determined by the Lyman-alpha line well visible in the red region of its spectrum (Photo 10b/00). The quasar is therefore located at a large distance that corresponds to when the universe was quite young, about 10% of its current age. The measured redshift was only slightly higher than what was predicted by the observers (z = 3.6) on the basis of earlier multi-colour photometric measurements from VLT/ANTU [2].
 
The magnitude of this quasar is 24.5, i.e., 25 million times fainter than the faintest star that can be seen with the naked eye at a dark site. As the observers remark, this quasar, at the measured magnitude and redshift, is an intrinsically fainter member of its class.
 
A good start
 
Another dozen objects also showed spectral features that will allow the Toulouse team to determine their distances, once their data have been properly analysed. The detection of these spectral features in such distant and faint objects is a powerful demonstration of the extraordinary sensitivity of the KUEYEN/FORS2 constellation. It is also a fine result from the very first observing night with this new facility and an good illustration of the effective use of space- and ground-based telescopes within the same research project.
 
The Toulouse team, with other colleagues, including Ian Smail (Durham University, UK) and Harald Ebeling (Institute for Astrophysics, Hawaii, USA), have again applied for observing time to continue this programme at the VLT, in order to measure the distances of multiple-lensed objects behind other massive clusters of galaxies observed with HST. With more observations of this type available, it will become possible to determine more accurately Omega and Lambda.
 
Notes
 
[1] The present project on the determination of cosmological parameters defining the geometry of the universe by means of multiple images that are gravitationally lensed by massive clusters of galaxies is carried out by a group of astr
onomers from the Observatoire Midi-Pyrenees (Toulouse, France), including Francisco Castander, Ghislain Golse, Jean-Paul Kneib and Genevieve Soucail.
 
[2] The photometric redshift method to determine cosmological distances is based on measurement of colours. Depending on the redshift and hence, the distance, distinct features in the spectra of galaxies produce changes in the observed colours. More information about the photometric redshift code HyperZ is available at http://webast.ast.obs-mip.fr/hyperz.
 
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