In the image of the Hubble Space Telescope, the two rings are clearly visible after the light of the foreground galaxy has been removed. (c) NASA, ESA, and R. Gavazzi and T. Treu
Reading Three galaxies lying exactly in line with each other form an unusual phenomenon in the sky: the gravitational force of the foremost galaxy distorts the light of the two underlying nebulae so that they can be seen as two concentric rings of different sizes ? like a kind of cosmic porthole. Such a "double Einstein ring" had never been observed before, researchers from Raphael Gavazzi and Tommaso Treu from the University of California at Santa Barbara now reported at the American Astronomical Society meeting in Austin, Texas. When two galaxies or even stars lie almost exactly one behind the other, the front object becomes a so-called gravitational lens: It deflects the light of the rear object in a similar way as a lens made of glass. Sometimes the image of the posterior galaxy is magnified by it, sometimes there are also several images. If both objects lie exactly on a line of sight, the rear object appears as a ring. The fact that they now have three galaxies lined up in a row like pearls on a chain, is for Treu as if he had "cracked the jackpot, " he reported at the meeting.

The discovery is also significant because the mass of the gravitational lens and that of the middle galaxy can be directly determined by the deflection of the light. Invisible dark matter is also weighed. The three galaxies are at a distance of three, six and eleven billion light years. With the middle galaxy it was for the first time able to determine the mass of a dwarf galaxy at such a large distance, said the researchers. The middle galaxy, seen as the smaller of the two rings, weighs as much as a billion suns.

If more of these double Einstein rings were to be found, the mass determinations could serve to determine the amount of dark matter present in the universe independently of previous estimates. The state of the even more enigmatic "dark energy, " which accelerates the expansion of the universe, could be accurately measured with the help of some 50 cosmic portholes. Team member Leonidas Moustakas of the Jet Propulsion Laboratory in Pasadena is excited about the find: "The elegance of this gravity lens is only surpassed by the secrets of nature it unveils."

Raphael Gavazzi (University of California, Santa Barbara): The Astrophysical Journal, filed by Ute Kehse advertisement

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