Communiqué de presse

Most Distant Cosmic Mirage?

22 octobre 1991

Observations with telescopes at the La Silla observatory have revealed that the image of an extremely distant quasar, known as Q1208+1011, actually consists of two images very close to each other. It is the most distant and also the brightest quasar ever observed to have a double image.

It is likely that this is a "cosmic mirage" which is caused by a gravitational lens (see the appendix) and that the two images are of the same distant object. If so, a strong gravitational field somewhere along the line of sight has bent the light from the quasar on its way to us. However, it cannot be excluded that what we see are two quasars, very close to each other in space. In both cases, the new discovery opens important possibilities for the study of the remotest regions of the Universe.

Searching for Cosmic Mirages

Quasars were first discovered in 1963 as a class of high-velocity objects with point-like images. Most astronomers now believe that they are the extremely energetic nuclei of distant galaxies. The amount of energy radiated by a quasar (its "absolute brightness") can be determined by measuring its apparent brightness as seen here on Earth and also its distance, since the intensity of the light measured decreases with the square of the distance.

However, the effect of gravitational lensing may not only produce multiple images, it may also concentrate and amplify the light of a quasar so that it appears brighter than it really is.

Since some years, an international group of astronomers [1] has been searching for gravitational lenses by studying selected quasars with a particularly high absolute brightness. Assuming that some of them appear brighter than they really are because of gravitational lensing, it is expected that upon closer inspection, some of the quasar images will turn out to be double or multiple.

Q1208+l011: as bright as 100 million million Suns?

The 17.5 magnitude quasar Q1208+1 011 in the northwestern corner of the constellation Virgo (the Virgin) was discovered in 1986; at that time it was the most distant known object in the Universe. With a redshift 2 of 3.8, corresponding to a distance of about 16,000 million light-years (if the Universe is 20,000 million years old), its absolute brightness was found to be no less than 100 million million times that of the Sun.

This is an incredibly high energy production. It is equivalent to the explosion of 100,000,000,000,000,000,000,000 (23 zeroes) one-megaton hydrogen bombs every second, or the complete transformation of one Earth mass into pure energy every 14 seconds. This immense energy could be produced by a heavy black hole at the centre of the quasar.

Suspecting that Q1208+1011 at least partly appears to be so bright because of gravitational amplification, the astronomers put it on their observing list, together with more than 150 other bright quasars. All of them have in the meantime been observed with high-resolution telescopes at La Silla and elsewhere; five have been shown to have multiple images and fourteen more are suspected. Gravitational lensing may therefore turn out to be a more common phenomenon than previously thought.

A first image of Q1208+1011 was obtained already in April 1987, but is was only in May 1991 that a thorough analysis indicated that it may be double, following the development of more advanced image processing software by the group.

Another image was obtained with the 2.2-metre ESO/MPI telescope at La Silla in early July 1991 and the analysis now clearly showed two images, separated by only 0.45 arcseconds. The brighter image is situated almost due north of the other and is about 3.5 times brighter. An American group has since obtained an image of Q1208+10ll with the Hubble Space Telescope during a "snapshot" survey of bright quasars. This image has been analysed by the international group immediately after public release; although it was obtained under less than optimal conditions (no telescope guiding), it fully confirms the ground¬based data.

Gravitational lensing or a binary quasar?

Q1208+1011 is the most distant quasar whose image has been found to be double. Its two images also have the smallest angular separation among the few known quasars with multiple images. But might it not be a binary quasar, that is two different quasars, which are very close to each other in space, and whose images are therefore seen in almost the same direction?

This can only be decided by obtaining individual spectra of the two images. If they are different, it must be a binary quasar. If they are completely identical, the two images are of the same object and they must be the effect of a gravitational lens. Such an observation, however, is exceedingly difficult because of the very small separation of the images.

The combined spectrum of Q1208+ 1011 shows many absorption lines, which are caused by hydrogen, carbon, silicon, aluminium, magnesium and iron in several galaxies, situated along the line of sight to Q1208+1011, approximately three-quarters of the way towards the quasar. One of them may cause the lensing effect.

The importance of Q1208+1011

The discovery of a double-image quasar at this enormous distance is very exciting and Q1208+10ll will now be further investigated with different techniques, including adaptive optics and speckle interferometry.

The astronomers intend to use the ESO New Technology Telescope at La Silla, the Nordic Optical Telescope on La Palma and the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, to obtain high-resolution images. This will hopefully allow to obtain individual spectra of the two images which will settle the question about the nature of Q1208+10ll.

If Q1208+1011 is a binary quasar, it provides us with a unique opportunity to study an exceedingly rare object (only one other is known), soon after its formation in the young Universe.

The two images will be monitored for intensity variations during the coming years. If it is a gravitational lens, as is now thought more likely, any brightness change in one image must be followed by a similar change of the second image after some time. The time interval accurately determines the difference in path length of the two light beams. From the observed separation of the two images and the ratio of distances of the gravitational lens and quasar, as deduced from their redshifts, it is then possible to determine the distance to the quasar in kilometres. This would permit an independent determination on a very large scale of the universal Hubble constant which describes the expansion and therefore the age of the Universe. Clearly, this will be an extremely important result.

It may also be that rapid brightness fluctuations will be seen which are caused by "microlensing" when compact objects move through the line of sight. In theory, this would allow the detection of planet-size objects, more than 12,000 million light-years way.

The light which we now observe from Q1208+1011 has been travelling towards us during more than 80 %of the age of the Universe. We may therefore be dealing with a two-fold mirage: the distorted image of an object that most probably no longer exists!

Notes

[1] The group consists of Jean Surdej, Eric Gosset, Damien Hutsemekers, Pierre Magain, lvlarc Remy, Jean-Pierre Swings and Eddy van Drom (Institut d'Astrophysique, Universite de Liege, Belgium), Marie-Christine Angonin, Laurent Nottale and Christian Vanderriest (Observatoire de Meudon, Paris, France), Jean Arnaud (Observatoire Midi-Pyrenees, Toulouse, France), Thilo Bauer and Gerd Weigelt (Max Planck Institut fiir Radioastronomie, Bonn, Germany), Ulf Borgeest, Rainer Kaiser and Sjur Refsdal (Hamburger Sternwarte, Germany), George Djorgovski (California Institute of Technology, Pasadena, USA), Olivier Hainaut and Alain Smette (ESO-La Silla), Olivier Le Fevre (Canada-France-Hawaii Telescope, Hawaii, USA), George Meylan (Space Telescope Science Institute, Baltimore, USA), Benoit Pirenne (Space Telescope-European Coordinating Facility, ESO-Garching), Peter Shaver (ESO-Garching), Mira Veron-Cetty and Philippe Veron (Observatoire de Haute Provence, France).

[2] The redshift denotes the fraction by which lines are shifted towards longer wavelengths in the spectrum of a distant galaxy or quasar receding from us with the expansion of the Universe. The observed redshift gives a direct estimate of the apparent recession velocity which is itself a function (known as the Hubble relation) of the distance of the object under study.

Plus d'informations

A detailed scientific account of the investigation of Q1208+10ll by P. Magain, J. Surdej, C. Vanderriest, B. Pirenne and D. Hutsemekers will appear in the European journal Astronomy & Astrophysics.

Appendix: gravitational lensing and amplification

Gravitational fields may act as optical systems of lenses and mirrors.

Since the total solar eclipse of 1919, when astronomers observed for the first time an apparent displacement in the positions of stars near the limb of the Sun, it is recognized that light beams can be bent, not only in optical systems, but also in gravitational fields. This effect was predicted by Albert Einstein within his General Theory of Relativity.

Bending of light is also observed when the light from a distant quasar passes close by one or more massive objects on its way to us. Such objects may be individual galaxies or clusters of galaxies. The effect is referred to as a gravitational lensing.

Depending on the form and intensity of the gravitational field, that is on the geometrical configuration and the masses of the objects in the gravitational lens, the light from the quasar may not only be bent into multiple images of the quasar, but some of these images may become brighter than the quasar itself would have appeared in the absence of the gravitational lens. This is referred to as light amplification.

Due to the amplification effect, we may be able to observe gravitationally lensed images of very distant quasars, which would otherwise have been too faint to detect with present telescopes. Gravitational lenses may therefore, at least in principle, allow us to investigate otherwise inaccessible, very remote regions of the Universe. It is also conceivable that gravitational lensing significantly perturbs our view of the distant Universe. Future observations may throw more light on this fundamental issue.

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Richard West
ESO EPR Dept
Garching, Germany
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