June 20, 2007 A
team led by Vasily Belokurov at the University of Cambridge in England has found the largest optical Einstein ring known. The astronomers say the object, which they dub the Cosmic Horseshoe, provides a unique laboratory for studying what the universe was like at one-fifth its present age.
An Einstein ring is a kind of cosmic mirage. Gravity from a foreground galaxy distorts light from a background galaxy into an arc. If the alignment between the background galaxy and the foreground "lensing" galaxy is precise enough, the distant galaxy's light becomes warped into a complete ring.
"Most Einstein rings were discovered in radio," Belokurov tells Astronomy
. "There were close to none in optical, until recently." Last year, astronomers used spectra from the Sloan Digital Sky Survey to select imaging targets for the Hubble Space Telescope's Advanced Camera for Surveys. The search netted eight of the roughly dozen optical rings known.
The SDSS survey can resolve a lensed ring into the multiple discrete images that compose it. Belokurov and his colleagues searched for multiple blue, faint companions around Luminous Red Galaxies (LRGs) in the SDSS catalog. Out came the Cosmic Horseshoe. "I didn't name it," Belokurov laughs, "but I like it."
The ring measures 10.2" across — more than 5 times the size of previously-known optical rings. With an arc of 300-plus degrees, it's also one of the most complete. "The image of the source is very bright and highly magnified because the lens is so massive," Belokurov adds.
The team followed up with images from the Isaac Newton Telescope on the Canary Islands and obtained the object's spectrum with the 6-meter BTA Telescope at the Special Astrophysical Observatory in Russia. The ring's spectrum shows its light comes from a star-forming galaxy at a redshift of 2.379, or when the universe was 2.8 billion years old.
"This epoch is very important and rather active," Belokurov explains. "The galaxies are put together and most of the heavy elements are produced at this time. Black holes are most actively accreting, and the number of luminous quasars peaks." Only now are astronomers beginning to gather information on objects at this redshift. "With this huge magnifying glass, we'll be able to study one in great detail," he says.
The lensing galaxy itself is an interesting object: It's a member of a rare population of LRGs. "These are the largest and most massive galaxies in the universe, and they're also believed to host massive black holes," Belokurov says.
The ring encloses some 6 trillion Suns, which makes the Cosmic Horseshoe's lens one of the most massive LRGs detected. "We don't really know the mass of the whole galaxy," the astronomer adds, "but it can be 10 times more." The lens lies at redshift 0.444, so astronomers see the galaxy as it was when the universe was 9 billion years old.
Astronomers believe galaxy halos harbor lots of dark matter — material that doesn't produce light and interacts with it only through gravity. At the lensing galaxy's distance, the ring spans nearly 200,000 light-years, so the new find also gives astronomers an opportunity to measure the distribution of dark matter in the lens' halo.
on the object will be submitted to The Astrophysical Journal