Light from a distant galaxy can be strongly bent by the gravitational influence of a foreground galaxy. That effect is called strong gravitational lensing. Normally a single galaxy is lensed at a time. The same foreground galaxy can—in theory—simultaneously lens multiple background galaxies. Although extremely rare, such a lens system offers a unique opportunity to probe the fundamental physics of galaxies and add to our understanding of cosmology.
One such lens system has recently been discovered—and the discovery was made not in an astronomer’s office, but in a classroom. The lens system has been dubbed The Eye of Horus, and this ancient eye in the sky will help us understand the history of the universe.
CLASSROOM RESEARCH PAYS OFF. Subaru Telescope organizes a school for undergraduate students each year. One such session was held in September 2015 at the National Astronomical Observatory of Japan (NAOJ) headquarters in Mitaka, Tokyo.
Subaru is currently undertaking a massive survey to image a large area of the sky at an unprecedented depth with the Hyper Suprime-Cam (HSC) as part of the Subaru Strategic Program. A group of astronomers and young students were analyzing some of that HSC data at the school, when they found this unique lens system. It was a classic case of a serendipitous discovery.
“When I was looking at HSC images with the students, we came across a ring-like galaxy and we immediately recognized it as a strong-lensing signature,” said Masayuki Tanaka, lead author of a science paper on the system’s discovery. “The discovery would not have been possible without the large survey data to find such a rare object, as well as the deep, high quality images to detect light from distant objects.”
Arsha Dezuka, a student who was working on the data, was astonished at the find. “It was my first time to look at the astronomical images taken with Hyper Suprime-Cam and I had no idea what the ring-like galaxy is,” she said. “It was a great surprise for me to learn that it is such a rare, unique system!”
WHAT THEY SAW. A close inspection of the images revealed two distinct arcs/rings of light with different colors. This strongly suggested that two distinct background galaxies were being lensed by the foreground galaxy. The lensing galaxy is 7 billion light-years away. Follow-up spectroscopic observations of the lensed objects using the infrared-sensitive FIRE spectrometer on the Magellan Telescope confirmed that there are two galaxies behind the lens, lying at 9 and 10.5 billion light-years away.
“The spectroscopic data reveal some very interesting things about the background sources,” said Kenneth Wong from NAOJ, the second author of the scientific paper describing the system. “Not only do they confirm that there are two sources at different distances from us, but the more distant source seems to consist of two distinct clumps, which could indicate an interacting pair of galaxies. Also, one of the multiple images of that source is itself being split into two images, which could be due to a satellite galaxy that is too faint for us to see.”
The distinct features for the system (several bright knots, an arc, a complete Einstein ring) arise from the nice alignment of the central lens galaxy and both sources, creating an eye-like structure. The astronomers dubbed it “Eye of Horus,” for the sacred eye of an ancient Egyptian goddess, since the system has an uncanny resemblance to it.
Eye of Horus in pseudo color. Enlarged image to the right show two arcs with different colors. The inner arc has a reddish hue, while the outer arc has a blue tint. These arcs are lensed images of the two background galaxies. There are blobs in and around the arcs, which are also the lensed images of those background galaxies. The yellowish object at the center is a massive galaxy 7 billion light-years away, which bends the light from the two background galaxies. (Credit: NAOJ)
Data analysis workshop for undergraduate students, held at the Mitaka Campus of the National Astronomical Observatory in September 2015. (Credit: NAOJ)
A schematic diagram showing the location of galaxies creating the gravitational lens effect of the Eye of Horus. A galaxy at 7 billion light-years from the Earth bends the light from the two galaxies behind it, one at 9 billion light-years, and the other at 10.5 billion light-years. (Credit: NAOJ)