Every now and then, the universe lines up just right for us puny humans on some speck of a planet in the no-where end of the galaxy to see something amazing. This is an Einstein Ring, a near-perfect manifestation of a particular variety of gravitational lensing.
Top image: Galaxy SDP.81 gravitationally lensed into an Einstein Ring. The bright ring is dust, with traces of carbon monoxide and water clouds seen in millimetre wavelengths. Credit: NRAO/ESO/NAOJ/B. Saxton/AUI/NSF
Albert Einstein's general theory of relativity included descriptions of how mass can warp space-time, with one of the many theoretical consequences being the ability of masses within our universe to act as giant gravitational lenses, bending and warping light. If, by sheer chance, the intervening mass is placed in exactly the right position compared to our planet, the lens will be perfectly symmetrical in bending light from an object behind it, creating a ring. Einstein lamented that we'd never observe a ring because they'd be too small for even his most optimistic dreams for the future of telescopes, but that's because he was only considering rings created by small, star-sized lenses. When galaxies or black holes mess take on the role of lenses, they can create massive Einstein Rings that are both easier to spot and reliably gorgeous.
Galaxy SDP.81 is about 12 billion light-years away, from when our universe was still sorting out how to birth stars and develop galaxy clusters. Between us and it is a relatively-near galaxy just 4 billion light-years away that, by sheer luck, has just the right mass and distance to act as a perfect gravitational lens and amplify the light from SDP.81. This allows us an otherwise-impossible look at the star-forming galaxy, helping us gain insights into how things have changed and how they've stayed the same in the intervening eons.
Composite image of SDP.81 in millimetre [orange] and optical [blue] wavelengths. Image credit: ALMA/NRAO/ESO/NAOJ/B. Saxton/AUI/NSF/NASA/ESA/Hubble/T. Hunter
While the image is distorted — SDP.81 is certainly not a perfectly circular galaxy resembling Sauron's Eye or the death-mark out of The Ring — it's distorted in a very specific way we can detangle with a bit of inversion mathematics applied to geometry and optics, allowing astronomers to recover the galaxy's original appearance. The end result is the capacity to image the original galaxy in truly outrageous detail for something so faint and so far away. The ALMA observatory press release explains in terrestrial terms:
For these observations, ALMA achieved an astounding maximum resolution of 23 milliarcseconds, which is about the same as seeing the rim of a basketball hoop atop the Eiffel Tower from the observing deck of the Empire State Building.
The highest-resolution image of SDP.81 taken by the ALMA observatory reveals the dust in 23 milliarcsecond resolution. Image credit: ALMA/NRAO/ESO/NAOJ