Perfectly Aligned Galaxies Found For the First Time

John Roach
for National Geographic News
January 11, 2008
Astronomers have found three galaxies in a never before seen perfect alignment—a discovery that may help scientists better understand the mysterious dark matter and dark energy believed to dominate the universe.

The three galaxies are like beads on a string, one directly behind the other, scientists announced yesterday at a meeting of the American Astronomical Society in Austin, Texas.

This makes the massive galaxy closest to Earth appear nestled in a pair of circular halos known as Einstein rings.

The phenomenon occurs because the gravity of the nearest galaxy distorts the light of the background galaxies as it travels toward Earth. The effect is called a gravitational lens.

When two galaxies are lined up perfectly, the distorted light forms a circle—a so-called Einstein ring. In this case, a third perfectly aligned galaxy forms a second, larger ring.

"Having three objects almost perfectly aligned along the line of sight is an extremely rare event," Tommaso Treu, an astrophysicist at the University of California, Santa Barbara, said at the press briefing.

"It's more unlikely than winning two consecutive bets in a single number of roulette."

Secrets of Nature

Not only is the optical alignment rare, but it also sheds light on some of the greatest mysteries of the cosmos, including dark matter and dark energy, Tommaso added.

"If you have two Einstein rings, you can start to measure a lot of things that you wouldn't be able to with just one ring," he said in a telephone interview Wednesday.

Dark matter, for instance, does not give off or emit light, so the substance can only indirectly be observed by the effects of its gravity. (Related: "Dark Matter Ring Detected by Hubble" [May 15, 2007].)

The double Einstein rings allow astronomers to calculate how dark matter is distributed in the galaxy closest to Earth by observing how the substance's gravity bends light from the more distant galaxies.

Such observations could help scientists pinpoint the nature of dark matter, which is believed to make up the vast majority of mass in the universe.

"[The dark matter is] distributed in a way that's different than the way visible matter is distributed—it's more extended; it's like a fuzzy halo around the stars," Tommaso said.

However, the distribution is not random. Rather, there is a relationship between the dark matter and the visible matter.

"The fact that we see this regularity tells us that there is some cross-talk between the two components," he said.

While previous research had suggested such a relationship may exist, astronomers still lack a simple explanation for it, Tommaso added.

Detecting Dark Energy

If researchers can find more double Einstein rings, they can compare the radius of the rings to better understand dark energy, Tommaso added.

Dark energy is a poorly understood force that is accelerating the expansion of the universe.

But understanding it is vital, since scientists calculate that there is about three times as much dark energy in the universe as there is dark matter and normal matter combined.

"The ratio of the diameters of the rings tells you something about how the universe expanded and so about dark energy," Tommaso said.

Meanwhile, the geometry of the two Einstein rings has allowed his team to measure the mass of the middle galaxy to about a billion times the mass of our sun. This indicates that the galaxy is a relatively small galaxy, known as a dwarf galaxy.

The dwarf is about six billion light-years away—the first dwarf galaxy accurately measured at such a distance, the researchers point out.

The nearest galaxy in the trio is about 2 billion light-years away from Earth, while the rear galaxy is about 11 billion light-years distant.

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