Call it the case of the missing dwarfs.
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.
The reason: The currently favored picture of galaxy formation, known as the cold dark matter theory, predicts that far more dwarf galaxies should be present than astronomers have seen as they hunt for starlight from the objects.
Cold dark matter is thought to comprise 90 percent of all the matter in the universe. It emits no light; its presence is inferred from the influence its gravity exerts on galaxies and clusters of galaxies.
Each galaxy is thought to have formed within its own vast cocoon of dark matter, then to have grown as it merged or collided with others.
The theory itself is on firm footing, Bullock explains. But since the theory seems to be pretty robust, it's predictions relating the distribution of numbers and masses of dwarf galaxies should also be robust.
Essentially, the theory allows researchers to derive an estimate of the number of dwarf galaxies that should be present at different masses. Dwarfs with the lowest masses, hence the faintest galaxies, should be the most numerous, Vegetti says.
So far, this aspect of the theory is more bust than robust at visible wavelengths. The Milky Way, for instance, should host thousands of dwarfs, she says, but so far, astronomers have found 30.
Vegetti's team tried an approach that doesn't look directly for a dwarf galaxy's starlight. Instead it takes advantage of gravitational lensing – the gravity from a massive galaxy bending and magnifying light from another object behind it. The galaxy acting as the lens may be obscured by intervening dust or gas. But astronomers know it's there by a tell-tale ring of light that forms around its location.
The team associates its dwarf galaxy with a large elliptical "lens" galaxy known prosaically as JVAS B1938+666. The dwarf showed up as an "excess" lensing effect at a unique spot along the ring of light surrounding the main galaxy. The team's unique approach to processing the data, gathered at the Keck Observatory on Hawaii's Mauna Kea volcano, allowed it to tease out the dwarf's mass.
Where galaxies like the Milky Way reside in vast halos of dark matter, dwarf galaxies are thought to exist within smaller "subhalos," like bubbles within bubbles. In some cases, the number of stars in a dwarf galaxy could be so few that the only way to detect it is through the gravitational influence of its dark-matter subhalo, some researchers suggest.
What if a more-exhaustive search fails to reconcile the gap in dwarf-galaxy numbers, especially for the smallest dwarfs?
"If they don't exist, then you have to change the properties of dark matter" to account for their absence, Vegetti says.