In October 2014, the Atacama Large Millimeter/submillimeter Array (ALMA) imaged a huge galaxy dubbed SDP.81. The galaxy (imaged below) sits a staggering 11.7 billion light-years from Earth in the constellation Hydra.
Credit: Y. Tamura (The University of Tokyo)/ALMA (ESO/NAOJ/NRAO) National Astronomical Observatory of Japan
Yep, that’s an Einstein ring. Another massive galaxy in the foreground acted as a natural telescope (called gravitational lens) to magnify the image of SDP.81. Note how the image becomes brighter, but the galaxy is distorted into a ring shape.
The galaxy in the foreground is much closer to Earth, located about 3.4 billion light-years away.
“ALMA was designed to be the most powerful telescope of its kind, but by harnessing the magnifying power of this gravitational lens we were able to study a distant and mysterious object in detail that would have been impossible otherwise,” said Todd Hunter, an astronomer at the National Radio Astronomy Observatory and co-author on one of the papers. “This one dataset has spawned an entire series of highly intriguing research, confirming that ALMA offers the astronomical community new avenues to probe the distant Universe.”
One problem that arises from this gravitational lensing effect is that it can be hard to decipher the details of the galaxy. University of Tokyo assistant professors Yoichi Tamura and Masamune Oguri worked alongside researchers at the National Astronomical Observatory of Japan to build a model to correct the lensing effects.
This model is the best one to date for gravitational lens and shows SDP.81 is a monster. The galaxy is churning out new stars at rates hundreds to thousands of times compared to what we see in our galaxy. Or, was. We are looking at light that took 11.7 billion years to reach us.
New model reveals inner structure of distant galaxy for the first time
The model showed researchers the inner structure of SDP.81. Several dust clouds ranging from size between 200 – 500 light years are located within an elliptic region that stretches 5,000 light-years across.
A reconstructed image of SDP.81 using the new models. Researchers do note that some of the smaller structures in the image could be artifacts created by the reconstruction. Credit: ALMA
Another cool feature of Einstein rings is that researchers can estimate the size of the central black hole in the foreground galaxy. If the foreground galaxy has a supermassive black hole, the central image of the Einstein ring should become fainter. The central image of SDP.81 is incredibly faint. Based off the faintness of the central image, researchers estimate the foreground galaxy’s black hole is over 300 million times more massive than our Sun.
Rob Ivison, ESO’s Director of Science and co-authors on a pair of papers, expressed his enthusiasm. “We can study galaxies at the other end of the Universe as they merge and create huge numbers of stars. This is the kind of stuff that gets me up in the morning!”
How gravitational lensing works
The general theory of relativity tells us mass curves the space around it. The same theory also predicted gravitational lensing. As light passes near a massive object, say a galaxy or cluster of galaxies, it bends slightly toward the mass.
If Earth is in just the right spot, we can observe distant objects with the help of a foreground galaxy. The image below illustrates how it works.
The target galaxy forms a ring structure around the foreground galaxy called an Einstein ring. A perfect Einstein ring has eluded researchers, but SDP.81 is one of the best Einstein rings ever imaged.
Here’s a bonus video of how gravitational lensing and the Einstein ring works.