A Near-Perfect Einstein Ring Imaged by ALMA

ALMA image of the gravitationally lensed galaxy SDP.81.

The Atacama Large Millimeter Array (ALMA) is fast becoming my favorite telescope. New results and observations are coming out as the array ramps up its technical capabilities, and this week the observatory released new images including a dazzling example of a nearly perfect Einstein ring of an active star-forming galaxy from the early universe. An Einstein ring is caused when the gravity of a massive foreground galaxy bends the light of the more distant galaxy. The background and foreground galaxy must be nearly perfectly aligned to form this symmetric shape.

The active galaxy in this ALMA image is SDP.81. The light has has been traveling from the galaxy since the universe was only 15 percent of its current age. The foreground galaxy that’s doing the lensing is much closer, about 4 billion light years away. The observation was made with ALMA when the antennae were at their maximum separation of 15 kilometers. This gives the maximum resolution, a resolution which exceeds that of all other telescopes that have observed the galaxy before, including the NASA/ESA Hubble Space Telescope. The resolution of this image is 23 milli-arcseconds. That’s about the angular size of a basketball hoop on top of the Eiffel Tower as seen from the Empire State Building. The ALMA image reveals fine structure in the Einstein ring that has never been seen before.

More about ALMA in this short video.