Warping Light From Distant Galaxies Is More Than a Pretty Halo
By DENNIS OVERBYE
Published: December 6, 2005
Just in time for the end of the Einstein year, astronomers have fetched from the sky a gallery of baubles: galaxies with pretty blue halos around them.
The halos, known as Einstein rings, are mirages. In each case, astronomers say, they are a result of light rays from a distant galaxy being bent around an intervening galaxy as in a lens. They are among the most elegant manifestations of gravity's ability to bend light, as decreed by Albert Einstein's general theory of relativity.
But they are not only pretty. They are also useful, allowing astronomers to weigh entire galaxies, said Adam Bolton, an astronomer at the Harvard-Smithsonian Center for Astrophysics. He is the leader of an international team that recently used the Hubble Space Telescope to discover and photograph eight of these rare mirages.
Dr. Bolton said he and his colleagues hoped to use the rings to study the distribution of mass in galaxies, including dark matter, which cannot be seen but is thought to comprise 90 percent of the universe.
Combining the ring data with other observations, he said, would allow astronomers to decompose the luminous and dark matter components of the galaxy.
''We can pick these galaxies apart really well,'' Dr. Bolton said.
Einstein's theory describes gravity as the bending of space-time away from Euclidean ''flatness,'' by matter and energy. It was the measurement of stars displaced outward from the Sun because of the bending of starlight around it during a solar eclipse in 1919 that ensconced Einstein and his theory in the world's consciousness.
A hundred or so gravitational lenses are now known in which a galaxy or cluster of them produces arcs or multiple images of a distant quasar. But for a perfect bull's-eye one galaxy must be lined up behind another at the right distance. This geometry is so rare that until last week only three complete Einstein rings were known.
To look for more, Dr. Bolton and his colleagues combed through data from 200,000 galaxies obtained in the Sloan Digital Sky Survey, an effort to measure colors and distances of about 100 million objects, including a million galaxies. They used the Hubble to examine 28 candidates. They found 19 gravitational lenses, including 8 rings.
''We went through hundreds of thousands of galaxies to find a 1-in-1,000 phenomenon,'' Dr. Bolton said, adding that they expected to find about 50 by the time the current Hubble observing cycle was complete.
The astronomers were looking for background galaxies with young blue stars haloing bright reddish elliptical galaxies. In the image at left, a foreground galaxy 2.8 billion light-years distant is ringed by light from a blue background galaxy 6.4 billion light-years away. The ring size is a measure of the amount of space-warping mass inside.
The uniformity of the sample, Dr. Bolton said, will allow the astronomers to address such questions as whether the relative amounts of dark and luminous matter are the same for galaxies of different size or whether the mix changes over cosmic time.
''It shows what's possible with the combination of a massive ground-based survey and the imaging capability of the Hubble,'' Dr. Bolton said. ''Our fondest hope is that this is a poster child for why we need something like the Hubble.''
The full power of the Sloan survey, which began in 1998, is just being realized, he said. ''For follow-up, we need the Hubble.''
Photo: GRAVITY'S POWER -- Light from a distant galaxy bends around an intervening one, forming a halo. The phenomenon helps astronomers weigh entire galaxies. (Photo by NASA/E.S.A.)