THE DOUBLE QUASAR, THE FIRST GRAVITATIONAL LENS TO BE OBSERVED

Looking at this stellar field there doesn’t seem to be anything special, some stars of the Milky Way in the first and a large number of galaxies very distant in space. In fact, despite their appearance, the two very bright objects located at the center of the image are not stars, but quasars, active galactic nuclei extremely bright and distant from Earth.

In the beginning, astronomers thought that this was the case, two separate quasars which, by coincidence, were almost on the same line of sight. In 1979, starting to study in detail the properties of the two objects, the researchers began to realize that something was wrong. The two objects seemed to have the same distance, the same appearance, the same spectrum and even the same physical properties! Such a coincidence was highly unlikely, and astronomers began to suspect that it was two images of the same quasar caused by a gravitational lens.

Until that moment gravitational lensing had only been theorized, but never observed. Initially, however, this hypothesis presented some problems. To obtain a gravitational lens like the one observed, it needs a very large amount of mass, much higher than that of the elliptical galaxy that is prospectively located between the images of the two quasars. All doubt was dispelled once it was discovered that this elliptical galaxy is part of a vast cluster of galaxies and it is precisely the latter, and not the single galaxy, that acts as a gravitational lens. So it was that, almost by chance, the first gravitational lens in history was observed!

Today we know that the system is composed of the elliptical galaxy (and its cluster) 3.7 billion light-years from Earth (z=0.355), while the quasar is 5 billion light-years further (z=1.4).

Credit: NASA, ESA, Hubble. 

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