Dwarf galaxies: breakthrough in bid to find 'fossils' of early universe
A team of astronomers reports that it has detected the most distant dwarf galaxy yet discovered orbiting an enormous elliptical galaxy some 10 billion light-years away.
Call it the case of the missing dwarfs.Skip to next paragraph
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Scientists have been searching the skies for dwarf galaxies – faint, ancient clumps of stars that merged early in the universe's history to form large galaxies such as the Milky Way.
Theory says lots of these dwarfs should still be around, like collections of spare parts, orbiting larger galaxies. But too few have appeared in astronomers' telescopes to match the theory.
Now, a team of astronomers reports that it has detected a dwarf galaxy orbiting an enormous elliptical galaxy some 10 billion light-years away.
This munchkin is the most distant dwarf galaxy yet discovered, and weighs in at an estimated 113 million times the mass of the sun – middling for a dwarf galaxy.
Researchers found it by detecting its gravitational influence on light from an even more-distant object – a result some astrophysicists call the first convincing evidence that this approach works for finding dwarf galaxies across such vast expanses of space.
The discovery, reported in the Jan. 19 issue of the journal Nature, shows that the process, known as gravitational lensing, "is going to be important as we try to flush this problem out," says James Bullock, an astrophysicist specializing in galaxy evolution at the University of California at Irvine.
At least two other groups have reported discovering dwarf galaxies using gravitational lensing, acknowledges Simona Vegetti, leader of the team that discovered the dwarf and physics fellow at the Massachusetts Institute of Technology in Cambridge, Mass.
The approach the others used allowed them to claim a detection, but the technique wasn't powerful enough to yield an estimate of the dwarf galaxy's mass or whether the system held more than one dwarf galaxy, Dr. Vegetti says.
Her team's approach allows estimates of both.
Dwarf galaxies don't have the eye-popping appearance of galaxies such as the Pinwheel or Andromeda galaxies.
Over the past several years, astronomers have found 25 newly identified dwarfs orbiting the Milky Way and the Andromeda galaxy. Most are dimmer than any previously known dwarfs. And a few are so dim they could be mistaken for globular clusters, typically collections of 1,000 or more stars whose mutual gravity keeps the group together.
Despite their lack of visual zip, dwarf galaxies have come to be recognized as fossils from the early epochs of galaxy formation.
Over the past decade or so, however, the hunt for these galactic no-seeums has gained momentum, explains Dr. Bullock, who was not part of the research team involved in the observation.