Two Rings

Now have a look at this object (and its enlargement on the right):

double einstein rings

What is it? It’s a double Einstein ring! An Einstein ring is formed by gravitational lensing – the bending of light from one object by the gravity of another object – and is typically formed when a distant galaxy lines up with another, closer galaxy. The result is a rather nice ring shape.

To find a double Einstein ring is rare! In fact, this is the first one that’s been announced. Not only is it novel, it can also use used to do a good deal of science, such as more accurate determination of the properties of the galaxies involved. Also, depending upon the type of and location of the galaxies in question, this can give us rather useful cosmological information as well, giving data on dark matter distributions for example, or even the geometry of the universe, giving valuable measurements independent of those we have now from other methods.

There’s a site at UCSB with a press release (since the team of astronomers on the Sloan Lens Advanced Camera for Surveys (SLACS) program was led by Raphael Gavazzi and Tommaso Treu of UCSB (the image above is from that team, released by NASA and ESA)), followed by the press release of the Space Telescope Science Institute (since Hubble was used for some of the work). They’re both worth reading. Link here. Extract:

The massive foreground galaxy is almost perfectly aligned in the sky with two background galaxies at different distances. The foreground galaxy is 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.


The distribution of dark matter in the foreground galaxies that is warping space to create the gravitational lens can be precisely mapped. Treu finds that the fall-off in density of the dark matter is similar to what is seen in spiral galaxies (as measured by the speed of a galaxy’s rotation, which yields a value for the amount of dark matter pulling on it), though he emphasizes there is no physical reason to explain this relationship.

In addition, the geometry of the two Einstein rings allowed the team to measure the mass of the middle galaxy precisely to be a value of 1 billion solar masses. The team reports that this is the first measurement of the mass of a dwarf galaxy at cosmological distance (red shift of z=0.6).

Here’s a bit about the cosmology one might hope to learn with more examples:

sample of several dozen double rings such as this one would offer a purely independent measure. The comparative radius of the rings could also be used to provide an independent measure of the curvature of space by gravity. This would help in determining the matter content of the universe and the properties of dark energy.

Observations of the cosmic microwave background (a relic from the Big Bang) favor flat geometry. A sample of 50 suitable double Einstein rings would be sufficient to measure the dark matter content of the universe and the equation of state of the dark energy (a measure of its pressure) to 10 percent precision.


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