Like a photographer taking a succession of random pictures of a crowd, the Hubble Space Telescope can also obtain selected, seemingly random images of different galaxies. When making these observations, Hubble's Advanced Camera for Surveys does not necessarily target one particular object; it can simply point toward an ordinary patch of sky.
In this image, taken in September 2003, Hubble's infrared camera was imaging a target near a region rich in galaxies. Visible in the field are a yellow spiral galaxy whose arms have been stretched by interactions with other galaxies (lower right), a young blue galaxy (top) glowing with young stars, and several small red galaxies. But the most remarkable feature is the beautiful blue arc in the middle of the image, looking almost like a photographic flaw.
The blue arc is actually a distant galaxy whose appearance has been transformed by the effect of gravitational lensing. This fascinating effect, first proposed by Einstein, occurs when light from a distant object is bent and stretched by an intervening mass. In this image, the gravitational lens—the intervening object—is the red elliptical galaxy about 6 billion light-years from Earth. Its red color indicates that it is composed largely of cool, older stars. The more distant object, whose image has been transformed into the blue arc, is about 10 billion light-years away. The light from this ancient galaxy dates from the first few billion years after the Big Bang, when the universe was about one-quarter of its present age. Its blue color indicates a young population of stars within the galaxy.
Gravitational lenses can be seen because the universe is rich in concentrations of galaxies. Light traveling through space from very distant galaxies may pass close to another galaxy along its path—rather like trying to make your way through a crowded intersection. But if the intervening galaxy is not massive enough, it cannot bend the light significantly. Long arcs like the one in this image are generally seen in massive galaxy clusters because of the enormous concentration of mass in those clusters.
Until this observation, such long arcs had not been seen beside individual galaxies, which makes the feature in this Hubble image unusual. For gravitational lensing to occur in this particular way, the galaxies must be almost perfectly aligned. Gravitational lenses provide valuable information about galaxies. These distinctive arcs offer an extremely useful way to determine a galaxy's mass directly and to measure the amount of dark matter it contains. Galaxies are not made only of stars, gas, and dust; an invisible form of matter known as dark matter accounts for much of their mass.