On Oct. 17, an international team of astronomers reported that they have found the most distant gravitational lens ever observed. A gravitational lens is a massive object, in this case a galaxy, that is capable of distorting spacetime to a sufficient extent to deflect or focus the light of a more distant object, in this case another galaxy.
Gravitational lensing is a result of general relativity, and provided one of the first experimental tests of the theory. The Sun's effect on the incoming light from stars located behind it from the perspective of observers on Earth led to the experimental confirmation of Albert Einstein's theory on May 29, 1919. (Such an effect was predicted by classical mechanics, but such predictions called for only half as much gravitational lensing.) The first galaxy that behaved in such a way was discovered in 1979.
Gravitational lenses consist of two objects: an object far from an observer that emits electromagnetic radiation, and an object closer to the observer that deflects the light. When the observer, the lensing object, and the emitting object are precisely aligned, the observer will see an Einstein ring, which is a perfect circle of light that is the projected and greatly magnified image of the distant light source.
“The discovery was completely by chance,” said Arjen van der Wal of the Max Planck Institute for Astronomy (Heidelberg, Germany), lead author of the study that appears in Astrophysical Journal Letters. “I had been reviewing observations from an earlier project when I noticed a galaxy that was decidedly odd. It looked like an extremely young galaxy, but it seemed to be at a much larger distance than expected. It shouldn't even have been part of our observing programme!”
Van der Wel studied images taken with the Hubble Space Telescope during its CANDELS and COSMOS surveys. The object looked like a galaxy with irregular features, and by combining available images and removing light noise, he was able to find an Einstein ring, which indicated an alignment between the lensing galaxy and the emitting galaxy within 0.01 arcseconds (2.778*10^-6 degrees). The light from the emitting galaxy has travelled 9.4 billion years to reach observers on Earth and has a redshift of z=1.53. The previous record was less than 8 billion light-years away and had a redshift of z=1.0.
The distortion caused by a lensing object allows for a direct measurement of its mass, including the mass of any dark matter which may be present. This allows astronomers to check their assumptions for distant galaxy masses, which are based on nearby galaxy masses.
The discovery, along with another like it, poses a challenge to current models of galaxy evolution. The precise alignment of lens and emitter should be rare, but there are now two examples of such an alignment involving starbursting dwarf galaxies, which are about one thousandth as massive as the Milky Way. Either astronomers looking for gravitational lenses are extremely lucky, or they are more common than once thought.
“This has been a weird and interesting discovery. It was a completely serendipitous find, but it has the potential to start a new chapter in our description of galaxy evolution in the early Universe,” van der Wal concluded.