eso8806 — Science Release
Discovery of a Cloverleaf Quasar in the Sky: A Lot of Hard Work - and a Little Bit of Luck
28 July 1988
Thanks to observations performed under near-perfect conditions at the La Silla observatory, it has been possible to show that the image of a distant quasar consists of no less than four components. Most appropriately, the object has now become known as the cloverleaf quasar. The peculiar image is due to the effect of "gravitational lensing", a phenomenon predicted by Einstein's General Theory of Relativity and explained at the end of this Press Release.
A rare gravitational lens
Quasars are known to be the most luminous objects in the Universe and can therefore be seen to distances greater than any other objects known. However, recent observations by a group of European astronomers at the ESO La Silla Observatory seem to indicate that the most powerful quasars could in fact appear more luminous than they actually are since their light may be amplified by the effect of gravitational lensing. Having found earlier that the quasar UM 673 (eso8715) has a double image due to this effect, they have now discovered that the image of another highly luminous quasar, known as H 1413+117, is in reality four very close images, which look like a four-leaf clover in the sky.
The images were obtained under very good atmospheric conditions with the ESO/MPI 2.2m telescope and a high-resolution CCD camera and show clearly the quasar H 1413+117 as a quadruple object. All four images are about equally bright and they are separated by less than 1 arcsecond; see the accompanying photo.
It was possible to obtain individual spectra of two of these images with the ESO Faint Object Spectrograph and Camera (EFOSC) at the ESO 3.6m telescope. The spectra are identical, indicating that the images are indeed of of the same quasar. The common redshift  is measured as 2.55, corresponding to an apparent recession velocity of about 85% of the speed of light. Because of the enormous distance that separates us from the quasar, we see it as is was 13 billion years ago. In addition, several narrow absorption lines are seen in the spectrum of one of the quasar images. These lines are thought to originate in matter associated with the galaxy that causes the lensing effect.
A continued astrophysical study of this rare gravitational lens system will contribute to our knowledge about the dark matter content of the Universe. It may also lead to an independent determination of important cosmological parameters, for instance of the age of the Universe.
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. As a matter of fact, this effect was predicted by 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 intensity and form of the gravitational field, that is on the mass and geometrical configuration 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.
 In astronomy, 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.
A detailed account of the observations and the interpretation of this gravitational lens system is contained in a scientific paper which will appear in the British journal Nature today (28 July 1988).
The group consists of P. Magain (formerly in Liège, now ESO, Garching), J. Surdej and J.P. Swings (Ins- titut d'Astrophysique, Université de Liège), U. Borgeest, R. Kayser and S. Refsdal (Hamburger Sternwarte, F.R. Germany), H. Kühr (Max Planck Institut für Astronomie, Heidelberg, F.R. Germany) and M. Remy (formerly in Liège, now ESO, La Silla).
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