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One Einstein ring to rule them all

Gravitational lensing has produced a spectacular ring from a starburst galaxy nearly 12 billion light-years from Earth.

The highest resolution image of SDP.81, where the bright region indicates the galaxy's glowing dust. ALMA (NRAO/ESO/NAOJ); B. Saxton NRAO/AUI/NSF

Located nearly 12 billion light-years from Earth is a galaxy called SDP.81. It's what is known as a "starburst" galaxy, named for its exceptionally high rates of stellar birth. This produces a lot of light, making starburst galaxies very easy to see in the sky.

Starburst galaxies can be any galactic shape -- but SDP.81 is special, appearing to the Earth-based Atacama Large Millimeter/submillimeter Array as a complete ring in the depths of space, in the most high-resolution image of the galaxy ever produced.

This is because directly in between SDP.81 and Earth, around four billion light-years from the latter, is another galaxy, large enough to bend the light coming from behind it, magnifying and distorting it -- lining up so precisely that it turns the light from SDP.81 into an almost perfect circle -- an effect known as an Einstein ring.

The combination of gravitational lensing's magnification and the strength of ALMA allow us to see SDP.81 in such detail. And because it is so far away, the light we see from the galaxy dates back almost to the beginning of the universe, which is estimated to be around 13.82 billion years ago -- distances that are usually too far to see.

"Gravitational lensing is used in astronomy to study the very distant, very early Universe because it gives even our best telescopes an impressive boost in power," said ALMA Deputy Program Scientist Catherine Vlahakis.

"With the astounding level of detail in these new ALMA images, astronomers will now be able to reassemble the information contained in the distorted image we see as a ring and produce a reconstruction of the true image of the distant galaxy."

ALMA/Hubble composite of SDP.81, where the lower-resolution portions indicate carbon monoxide, and the blue glow in the centre is caused by the lensing galaxy. ALMA (NRAO/ESO/NAOJ); B. Saxton NRAO/AUI/NSF; NASA/ESA Hubble, T. Hunter (NRAO)

Taken in October last year as part of an ALMA program to test and verify the telescope's highest resolving power -- achieved when the antennas are at their highest separation, 15 metres apart -- the highest resolution image was made by observing the bright light emitted by the galaxy's cosmic dust, creating a circle that is almost complete.

Other details observed by ALMA in images at slightly lower resolutions include molecular signatures that indicate carbon monoxide and water, providing important details about the composition of this active early-universe galaxy -- details that had not been picked up by other telescopes, such as Hubble and the Mauna Kea Submillimeter Array, studying SDP.81.

ALMA's maximum resolution is 23 milliarcseconds -- the equivalent, the ALMA team said, to seeing a basketball hoop on the Eiffel Tower in Paris, France from the observation deck of the Empire State Building in New York, the US.

"The exquisite amount of information contained in the ALMA images is incredibly important for our understanding of galaxies in the early Universe," said Charlottesville, Va. National Radio Astronomy Observatory astronomer Jacqueline Hodge.

"Astronomers use sophisticated computer programs to reconstruct lensed galaxies' true appearance. This unravelling of the bending of light done by the gravitational lens will allow us to study the actual shape and internal motion of this distant galaxy much more clearly than has been possible until now."