Astronomers spy rare double 'Einstein ring'
Photo: NASA, ESA, R. Gavazzi and T. Treu (University of California, Santa Barbara), and the SLACS team
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.
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I have some problems with these Einstein ring images I’ve been seeing. First of all, this one and the others with thin bluish rings around them don’t look anything like B1938+666.
Second, why are the arcs blue? Every galaxy I see at greater and greater distances is more and more red.
Third, I’d like to know the method used for determining the distances to these three galaxies. How much red shift was observed in the nearest one and what about the others?
Fourth, how come the only images we have are of galaxies billions of light years away? Why don’t we see this lensing around any nearby galaxies? One would think it much more likely and much more easily detected.
Finally, despite the presumed existence of dark matter, wouldn’t these arc radii virtually overrule the possibility that they are lens effects? We’re talking about tens (hundreds?) of thousands of light years from the center of gravity and the mass, even if packed with dark matter, would not have NEARLY enough gravity at that distance to cause such lensing. Around black holes? Sure. Extremely close to stars? Sure. But isn’t the gravitational strength of these objects at those distances billions or trillions of times greater than they would be in even the most optimistically massive cluster of galaxies,(including their supposed dark matter)? They totally lack the needed density, don’t they?
Any explanation or link would be appreciated.
B. Watson