eso8712 — Science Release
Discovery of a Binary Quasar
15 July 1987
The discovery of what may be the first true binary quasar has been reported by a European-American team of astronomers using a combination of optical, spectral, and radio observations. The pairs of nearly identical quasars, separated by only 4.2 arcseconds projection on the sky, have a redshift of 1.345, corresponding to a distance of some 12 billion light-years from Earth (according to the standard cosmological distance scale) and are apparently associated with the radio source PKS 1145-071 in the constellation Crater .
The radio source PKS 1145-071 was known for many years to be associated with the quasar, but its binary nature had not been noted. Images of the object obtained 29 December 1986 at the European Southern Observatory's 2.2-meter telescope at La Silla, Chile, confirmed suspicions that the source was a double object. Spectroscopic observations to ascertain the nature of the two faint star-like components were obtained on 3 and 4 January 1987, at the Multiple Mirror Telescope Observatory in Arizona, operated by the Smithsonian Institution and the University of Arizona. The spectra confirmed that both objects (denoted A and B) were indeed quasars, very similar, and at essentially the same distance from Earth.
At first, the astronomers thought that the twin images might be another example of the so-called gravitational lens phenomenon: an image of a single distant quasar split in two by the gravitational field of a Galaxy or cluster of galaxies lying between the observer and the quasar. First predicted by Einstein's general theory of relativity some fifty years ago, several such gravitational lenses are currently known, the first one of which was, coincidentally, also confirmed by the Multiple Mirror Telescope in 1979.
Closer examination of the spectra of the two quasars revealed some subtle differences, reflecting slightly different physical conditions in the quasars themselves. Moreover, there appeared to be a small, but measurable relative velocity difference between them. This suggested that the pair may be two distinct quasars, rather than lensed images of a single object. However, the gravitational lens hypothesis could not be discarded on the basis of such data alone. Quasars are relatively rare in the universe, and having two of them so close together would be unprecedented.
The crucial observations which confirmed the physical binary nature of the system involved radio-wave imaging of PKS 1145-071 with the Very Large Array (VLA) of radio telescopes near Socorro, New Mexico, on 9 January 1987. The data showed only one radio quasar in the field, corresponding to the optically brighter quasar. This result was contrary to the gravitational lens hypothesis since a lens should split images equally in both visible light and radio waves. The conclusion was that the two quasars represented a true physical pair, near each other in space and possibly interacting or even in a collision.
The discovery of quasars, about 25 years ago, was one of the most exciting events in the history of modern astronomy. Despite of the slow growth in understanding their physical nature, these objects, which are the most distant known, still provide the best available probe of the most remote observable regions of the universe.
A true binary quasar can offer astronomers important clues to the origin and the maintenance of quasar activity, which is often thought to be caused by collisions of galaxies. The light from the two quasars can be also used to measure the sizes of the intergalactic gas clouds lying between them and Earth.
From the measured velocity difference and projected separation of the two quasars, the astronomers estimated the pair may have a mass at least equal to that of some hundred billion solar masses. (Although commonly used for the binary stars in our galaxy, this is the first time such measurement has been possible for quasars). Such masses are typical for normal galaxies, and this measurement supports further the generally accepted interpretation of quasars as active nuclei of distant galaxies.
It is also possible that the two quasars are members of an extremely distant cluster of galaxies. Mere existence of rich clusters of galaxies at such large redshifts is an interesting constraint for the theories of large-scale structure formation, and the future studies of normal galaxies in this hypothetical cluster could be extremely valuable for the studies of galaxy evolution in the early universe.
This Press Release is accompanied by a photo showing the radio and visible-light images of QQ 1145--071.
 The text of this communication was released simultaneously by the Harvard-Smithsonian Center for Astrophysics, Boston, Mass., U.S.A.
The team reporting this discovery includes S. George Djorgovski (Harvard-Smithsonian Center for Astrophysics), Georges Meylan (European Southern Observatory), Richard Perley (National Radio Astronomy Observatory), and Patrick McCarthy (University of California at Berkeley). A preliminary announcement appeared in the June 1987 issue of ESO's journal The Messenger. The full report will appear in the Astrophysical Journal Letters.
Tel: +49 89 3200 6276