Many travelers on the world's deserts succumbed to thirst and heat while slogging toward elusive lush mountains that appeared to be seductively close. But what they were seeing were the phantasms of ranges many miles distant, projected by a lens of heated air that acted like a giant magnifying glass.
In 1936 Albert Einstein computed that the immense gravity of massive objects in space--galaxies and black holes--would bend light in a similar fashion to heated air, creating mirages of even more distant objects in the cosmos. But with the ground-based telescopes of the day, he remarked, "there is no great chance of observing this phenomenon."
The resolving power of modern telescopes has exceeded Einstein's expectations; the first gravitational lens was discovered in 1979 from ground-based observations. Now, with the vastly superior resolving power of the Hubble Space Telescope, astronomers are discovering that the cosmos is rife with these windows on the most distant reaches of the universe. In the May issue of the Astronomical Journal, astrophysicists Kavan Ratnatunga and Richard Griffiths and their colleagues at Carnegie Mellon University in Pittsburgh report that a quick survey of 500 Hubble fields of sky uncovered a significant number of rings, arcs and crosses apparently produced by gravitational lenses.
The Carnegie Mellon team identified a "Top 10" of the most interesting lens candidates in the deepest--or most distant--100 Hubble fields obtained by one of the space telescope's most sensitive instruments, the Wide Field Planetary Camera. Three of these images are shown at the left. The lensing galaxies are yellow to red; the mirages surround them with an ethereal blue-white arc or multiple images. (Click on the images for more details.)
The findings represent a significant increase in the number of known optical gravitational lenses. Hubble's sensitivity and high resolution allow it to see faint and distant lenses that cannot be detected with ground-based telescopes whose images are blurred by Earth's atmosphere. The fields examined by the Carnegie Mellon researchers cover a total area equal to only that of the full moon.
Just what the lenses are showing is another problem, however. The lens may produce multiple or distorted images of the background object as seen by the observer. Seven of the team's candidates are "strong lenses," which appear to have multiple images of the source. Three are cases where the single image of the source galaxy has been distorted into an arc. "Follow-up spectroscopic observations are now needed to verify that the object is far more distant than the lensing galaxy, as well as to derive better distance estimates to confirm that multiple images really belong to the same object," says Ratnatunga.
Even so, the Hubble's ability to see so many of these lenses in a small fraction of the sky takes them from being a scientific curiosity to serving as a potentially powerful tool for probing the universe's evolution and expansion. Astronomers hope that the Advanced Camera for Surveys, scheduled to be installed on Hubble next year, will be able to discover many more gravitational lenses because of its sensitivity and wider-angle coverage.
Instead of luring unwary travelers to their doom, these cosmic mirages may turn out to be polestars guiding a new generation of explorers to the origin of the cosmos.