Steward Observatory Aids Discovery of Eye of Horus
Named for the sacred eye of an ancient Egyptian god, the object is the first gravitational lensing system with a galaxy lens in which the distances to two background galaxies have been measured accurately.

University Relations – Communications
July 26, 2016


The inner arc of the Eye of Horus has a reddish hue, while the outer arc has a blue tint. The yellowish object at the center is a massive galaxy that bends the light from the two background galaxies.
The inner arc of the Eye of Horus has a reddish hue, while the outer arc has a blue tint. The yellowish object at the center is a massive galaxy that bends the light from the two background galaxies. (Image: National Astronomical Observatory of Japan)

Light from a distant galaxy can be strongly bent by the gravitational influence of a foreground galaxy, an effect that is called strong gravitational lensing.  Normally a single galaxy is lensed at a time, but in theory the same foreground galaxy can simultaneously lens multiple background galaxies. Though 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, recently discovered by a team of undergraduate students and researchers at the National Astronomical Observatory of Japan and at the Steward Observatory at the University of Arizona, has been dubbed the Eye of Horus.

In this system, two distant galaxies are gravitationally lensed by one single foreground massive galaxy (so-called double source-plane system). The mass of the foreground galaxy bends the rays of light coming from the background galaxies — a well-known effect explained by Einstein's Theory of General Relativity, which describes the nature of gravity. As a result, the foreground galaxy acts like a lens, bending/stretching the images of the background galaxies and producing ring/arc-like image structures around the foreground lensing galaxy.  This creates an image on the sky that resembles a human eye.

The Eye of Horus, named for the sacred eye of an ancient Egyptian god, is the first double source-plane system in which the distances to the two background galaxies have been measured accurately. Based on data from the Sloan Digital Sky Survey, the lensing galaxy lies 7.0 billion light-years away, while the two background galaxies are 9 and 10.5 billion light-years from Earth, respectively. The paper reporting this discovery has been accepted for publication in the Astrophysical Journal Letters.

The discovery images taken with Hyper Suprime-Cam on the 8.2-meter Subaru Telescope on Mauna Kea, Hawaii, clearly showed two concentric arc/ring-like structures with different colors around a bright foreground galaxy, indicating that this probably is a rare double source-plane system. The distance to the foreground galaxy was known previously because it is a bright galaxy. However, the two background lensed galaxies are faint and were discovered by Subaru for the first time.

"Although measuring distances to such faint galaxies is always a challenge, we were able to accomplish this goal by using one of the twin 6.5-meter Magellan Telescopes in Chile with the near-infrared spectrograph called FIRE," said Eiichi Egami, an astronomer with the Steward Observatory.

"After having been notified of the discovery of this system in fall 2015, we quickly applied for the use of unassigned Magellan nights in February 2016, which produced these results. Steward's excellent access to large telescopes like Magellan was the key to the quick publication of this paper. Without our contribution, we would have had to wait for another year to obtain the necessary data.

The large ongoing imaging survey with Hyper Suprime-Cam, a brand-new optical wide-field camera on Subaru, is only 30 percent complete, and it will keep collecting data for several more years. However, astronomers expect to find only about 10 more such lens systems in the survey, which highlights the importance of this first discovery.

"It's always hit-and-miss when it comes to measuring distances to faint galaxies, even with large telescopes like Magellan," Egami said. "We need to be lucky enough to see multiple strong emission lines produced by ionized gas in the target galaxy in order to determine the distance uniquely. So we were very excited at the telescope when we saw a few bright emission lines popping up in the spectra before our eyes. Such a moment of discovery, small or large, excites astronomers and keeps them going."

In collaboration with the Subaru/Hyper Surpime-Cam team, the Steward Observatory is currently conducting a complementary near-infrared imaging survey using the United Kingdom InfraRed Telescope, or UKIRT, also on Mauna Kea. UKIRT is another telescope with which Steward now has a large share of observing time, and the Steward-Subaru collaboration is expected to produce a variety of similarly exciting discoveries in the coming years.


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Eiichi Egami

UA Steward Observatory