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Double bull's-eye for Einstein

Using the Hubble Space Telescope, astronomers say they have spotted their first double Einstein ring – a bizarre optical phenomenon that shows how massive objects like galaxies can bend light rays, furnishing evidence for Einstein's general theory of relativity.

NASA / ESA / UCSB
The gravitational-lens system known as
SDSSJ0946+1006 includes a bright foreground
galaxy at center, the ringlike image of a
middle galaxy 6 billion light-years away,
and the dimmer ring of another galaxy
11 billion light-years away.


The fact that there's a double ring around this gravitational-lens galaxy means that two other galaxies are aligned precisely behind it. And the odds of that happening are estimated at 1-in-10000. That's a big reason why Tommaso Treu of the University of California at Santa Barbara felt as if he and his colleagues "hit the jackpot" when they saw the double ring's signature in data from the Sloan Digital Sky Survey.

Single Einstein rings are rare enough: On a telescope image, such a ring looks like a faint circlet of light surrounding a massive galaxy. The circle is actually the light from a galaxy much, much farther away, which has been bent around the closer galaxy to provide a distorted image.

This diagram shows how the closer galaxy serves as a lens to twist the light beams like a funhouse mirror - demonstrating that light beams are affected by gravitational fields, just as Einstein said they were. Our "Putting Einstein to the Test" interactive explains how gravitational lensing and other strange-but-true concepts relate to general relativity.

Over the years, Treu and the other astronomers involved in the Sloan Lens ACS Survey have spotted a gaggle of Einstein rings - but the ring-hunters suspected that they had something special when they happened upon the gravitational-lens system known as SDSSJ0946+1006.

"The original signature that led us to this discovery was a mere 500 photons hidden among 500,000 other photons in the SDSS spectrum of the foreground galaxy," Adam Bolton of the University of Hawaii's Institute for Astronomy said in a news release put out during this week's meeting of the American Astronomical Society.

The double ring was clearly visible in Hubble imagery of the same spot - telling the astronomers that two galaxies were both exactly behind the massive foreground galaxy.

"When I first saw it, I said, 'Wow, this is insane!" Treu said. "I could not believe it!"

The team analyzed the geometry of the two rings to determine how far away the galaxies were: The foreground galaxy is about 3 billion light-years away, the middle galaxy is 6 billion light-years away, and the farthest-out galaxy is 11 billion light-years away - which would put it close to the frontier of the observable universe. Astronomers could even calculate the mass of the middle galaxy at 1 billion solar masses, representing the first such measurement of a dwarf galaxy at cosmological distances.

A research paper on the findings has been submitted to The Astrophysical Journal.

Einstein rings make for much more than mere pretty pictures: An analysis of the ring's geometry can reveal how much mysterious dark matter the gravitational-lens system contains.

"Dark matter is not hidden to lensing," Leonidas Moustakas of NASA's Jet Propulsion Laboratory said in Thursday's news release. "The elegance of this lens is trumped only by the secrets of nature that it reveals." 

For SDSSJ0946+1006, the researchers estimate that dark matter makes up 66 to 82 percent of the system's mass - which is in the right ballpark, based on other observations.

If astronomers can find enough of these double rings, they could even run a statistical analysis to arrive at an independent, more precise measure of how gravity affects our space-time continuum. The studies so far indicate that our universe is geometrically flat rather than curved, with dark energy providing an accelerating push to cosmic expansion.

A sample of, say, 50 double rings would provide a better fix on the dark matter content of the universe as well as the influence of dark energy. The researchers note that a couple of space missions now under consideration, America's Joint Dark Energy Mission as well as Europe's Dark Universe Explorer, could provide just that kind of data - not to mention more glorious double-circlets to stare at.