eso8716 — Science Release
The Nature of the Mysterious “Luminous Arc” Revealed: A Gravitational Einstein Ring
5 November 1987
The prototype of a new class of astrophysical phenomena has now been interpreted. Studies of other objects of the same type will open entirely new vistas in the exploration of the nature of distant galaxies.
The combination of a very dark sky at the ESO La Silla observatory, the large collecting area of the 3.6 m telescope and the high efficiency of the ESO Faint Object Spectrograph (EFOSC) has made it possible to obtain for the first time a convincing spectrum that reveals the true nature of a mysterious object, the prototype “Giant Luminous Arc" in the distant cluster of galaxies Abell 370.
Several atomic spectral lines have been unambiguously identified in the arc by a group of astronomers from the Toulouse Observatory, France. They were able to show that the object is a “gravitational ring", a phenomenon predicted by Einstein, but never observed before. It is caused by the deflection of light from a background galaxy by the dense core of the cluster Abell 370. This exciting discovery opens an entirely new field of observational astronomy.
The giant arc was first recognized by the French group in September 1985, on direct images of Abell 370, obtained with the French-Canadian 3.6 m telescope on Mauna Kea, Hawaii . The distance to this cluster of galaxies is about 4600 million light years (the redshift is z = 0.374) and the apparent size of the visible segment of the arc is at least 500 000 light years.
After the subsequent discovery of another arc in the cluster of galaxies Cl2244-02 by American astronomers later in 1986, several theories were advanced about their nature. One possibility was that the two arcs were the results of gravitational lensing by the dense cluster of galaxies (see also eso8712). According to Einstein's Theory of Relativity, light rays can be bent by a gravitational field. An observer may then see multiple images or even an arc, depending on the relative positions of the foreground mass and the background object.
A determination of the true nature of the arcs had to await observations of their spectra, a task that had been viewed as almost impossible because of their faintness. However, for the first time, the French team has now been able to obtain spectra of the various segments of the giant arc in Abell 370. This shows that the spectrum is the same in different parts of the arc and also that it is the spectrum of a distant galaxy. Thus the arc is indeed a “gravitational ring".
The observations were made with the EFOSC/PUMA2 spectrograph at the Cassegrain focus of the ESO 3.6 m telescope, on October 18 - 22, 1987. Several 90 minutes exposures were made through a long and a curved slit, centered on the arc. In total, 6 hours of integration time was needed to bring out the rather noisy spectrum, attached to this Press Release. The spectrum shows several identifiable lines, including a comparably strong emission line from ionized oxygen, all shifted towards the red part of the spectrum. The measured redshift is 0.724 and the overall intensity profile of the spectrum is that of a galaxy at this redshift. This corresponds to a distance of about 7500 million light years; that is almost twice as distant as the galaxies in Abell 370.
The light of the distant galaxy is obviously deflected by the central core of the cluster Abell 370. It is confirmed that the central part of the arc, as well as a feature at the eastern end, both belong to the gravitational ring.
The creation of such a ring demands a rather specific geometric relationship between the background galaxy and the cluster, as well as a specific mass distribution within the cluster, of which studies of the ring's geometry and brightness will give detailed information. This kind of research is useful for verification of our understanding of gravitational theory and galaxy mass distributions. Since in certain configurations the arc-shaped image may be amplified, it is conceivable that distant clusters of galaxies may be used as “gravitational telescopes" to search for very distant objects in the Universe.
With the EFOSC instrument at the ESO 3.6 m telescope, it is possible to search efficiently for other gravitational rings, by obtaining direct images of other distant clusters of galaxies. Due to the highly efficient instrumentation, each exposure can be made in a few minutes only and several hundreds of clusters may be surveyed during a few nights.
The French astronomers have also made EFOSC observations of the only other known arc in the cluster Cl2244-02, now in the process of being reduced.
This Press Release is accompanied by a graphic representation of the spectrum of the central part of the arc, as obtained with the EFOSC/PUMA2 instrument, attached to the ESO 3.6 m telescope. It shows the redshifted, strong emission line of singly ionized oxygen, as well as weaker absorption lines of hydrogen and ionized magnesium. The break at rest wavelength 4000 Å is near two strong absorption lines of single ionized calcium.
 A picture of the arc has been reproduced in June 1987 ( 48, page 44) issue of the ESO Messenger. It is also available from the ESO Information Service upon request.
The complete discussion of the exciting observations of the two arcs will be published in the European journal Astronomy & Astrophysics.
G. Soucail, Y.Mellier, B. Fort, G.Mathez and M. Cailloux.
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