Rare double Einstein ring explained

In early 2008, astronomers announced a double Einstein ring.

Tommaso Treu: An Einstein ring happens when you have two objects perfectly aligned along a line of sight.

That’s Tommaso Treu of the University of California, Santa Barbara. He’s describing a single Einstein ring, which really looks like a ring in space. He and his colleagues were surprised to find a double ring – one ring inside the other.

Tommaso Treu: In order to have a double ring, you need to have an extraordinary coincidence of three objects aligned along the line of sight, and this is very rare.

This object is called SDSSJ0946+1006. So there are thought to be three objects here – a distant galaxy that warps the light of two more galaxies exactly aligned behind it. Treu’s team searches for Einstein rings.

Tommaso Treu: And we have a procedure to do this. It’s pretty much finding needles in a haystack.

They used the Hubble Space Telescope to find the double ring. Einstein’s theories of light and gravity help explain these objects, which are said to undergo an effect called “gravitational lensing.”

Tommaso Treu: If you have a concentration of mass, it produces a perturbation of space-time. If there is a big mass, it bends the trajectory of light rays. So we see light rays curving. If you have enough mass, light rays can curve and turn around the mass. So you see more than one image of the system.

The odds of seeing a double Einstein ring are thought to be about 1 in 10,000.

The foreground galaxy in the SDSS J0946+1006 system is thought to be located 3 billion light-years away. The inner ring and outer ring are comprised of multiple images of two galaxies at a distance of 6 billion and approximately 11 billion light-years. These objects are exactly aligned as seen from our earthly vantage point, one exactly behind the other. That special alignment creates the image of a ring in space.

Treu explained further that it’s not the same ray that turns around a mass that creates multiple images of the same source. Instead, it’s two different rays, with two different bending angles.

Our thanks to:
Tommaso Treu
University of California
Santa Barbara, CA