Astronomers using the Hubble Space Telescope report an incredibly lucky sighting of a young star-forming galaxy. The alignment of the normal galaxy and the distant galaxy had to be nearly perfect in order to create the gravitational lensing observed by the astronomers.

Gravitational lensing
The nearer galaxy is in the center and the distant starburst dwarf galaxy is seen as a ring of images, known as an "Einstein Ring." NASA/ESA/A. van der Wel

Gravitational lensing, first predicted by Albert Einstein, states that light from one galaxy will be bent, or distorted, by gravity. In this case, the light from the young "starburst dwarf" galaxy is being distorted as it passes by a nearer galaxy. The first gravitational lens was discovered in 1979, confirming Einstein's earlier theory, and giving researchers a valuable tool to observe galaxies, the effect of gravity and, indirectly, dark matter.

The gravitational lensing observed by lead author Arjen van der Wel of the Max Planck Institute for Astronomy, Heidelberg, Germany, and his team is the most distant yet as a nearer galaxy distorted light from a starburst dwarf galaxy whose light took 9.4 billion years to reach Earth. A starburst dwarf galaxy is a young galaxy that is experiencing an increased amount of star formation activity. The study was published in the Astrophysical Journal Letters.

The photo captured by Hubble reveals an "Einstein Ring," described as a "a perfect circle of light that is the projected and greatly magnified image of the distant light source." In the image, the nearer galaxy is in the center while the projected light forms a ring around it, an extremely rare phenomenon according to the researchers. Gravitational lensing is useful to astronomers as it allows for the measurement of a galaxy's mass as well as dark matter, which does not emit light but can be measured by its gravitational effects, such as the distortion of galaxies.

While the gravitational lens may have been "lucky" and helped astronomers to test current methods used to measure the mass of galaxies, it has led to some new questions. According to the researchers, the chances of a starburst dwarf galaxy being gravitationally lensed should be a rare phenomenon, but it is the second such galaxy to have been observed by this method.

The researchers are looking into the possibility that starburst dwarf galaxies in the early universe may be more common than previously believed, but that will require further observation. "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," said van der Wel in a statement.

Gravitational lensing was recently used to observe the jet from a supermassive black hole at the center of PKS 1830-211.