Giant Gas Loops Found in Center of Milky Way, Scientists Say

Sean Markey
for National Geographic News

October 5, 2006

Two giant loops of cold, dense molecular gas have been seen in the central region of our galaxy, according to researchers based in Japan.

The team says that the features extend for nearly a thousand light years and each loop has a mass about 800,000 times that of our sun.

The astrophysicists believe that the loops are the product of the same disturbances in magnetic fields that cause solar flares—plasma loops and arches that shoot from the sun's surface—but on a galactic scale (related photos: solar storms).

"We interpret these features as magnetically floated loops within the nuclear disk, analogous to … solar loops … ," the team writes in a paper that will appear in tomorrow's issue of the journal Science.

Experts have theorized about the presence of such formations in the Milky Way and other galaxies for nearly 40 years.

The new findings by Nagoya University astrophysicist Yasuo Fukui and colleagues mark the first time that scientists appear to have directly observed the features.

Magnetic Instability

Mark Morris, an astrophysicist at the University of California, Los Angeles, says that other researchers have previously reported seeing looplike structures in the galaxy's center.

(Related news: "New Planet 'Bonanza' Discovered at Center of Milky Way" [October 2006].)

But the observations were most likely optical illusions created by bubbles blown in the mix of plasma, gases, and dust that pervades the galaxy, he says.

Such bubbles could be formed by stellar winds or by blast waves from a supernova—the explosive death of a large star.

Morris, who wrote an accompanying perspective article on the findings for Science, believes that the Japanese team's findings are legit.

"These particular loops have never been seen [before]. They've only been known about theoretically," he said.

"Nobody has convincingly identified any galactic loops [until now] … and that makes them quite interesting."

Fukui and his colleagues suggest that the loops they observed are the product of a particular kind of disturbance known as the Parker instability.

This type of disturbance, which is known to occur on the sun, would be responsible for the magnetic buoyancy that supports the gas loops.

Morris, the UCLA astrophysicist, says that any number of cosmic events can initiate the disturbance.

In theory, an event such as a star's explosion can start a ripple in the magnetic field lines that run parallel to the galactic plane.

As the once-straight lines begin to undulate like waves, gas will slip from the peaks and begin to pool in the troughs.

The depressions get even deeper as higher concentrations of gas are pulled downward by the gravitational force of stars in the central plane of the galaxy.

Likewise, as gas flows into the troughs, weight is lifted from the magnetic field's peaks, which can then expand like a balloon.

The Parker instability would explain the loops of gas extending above the galaxy's center (the peaks) and the dense clouds of gas that pool where the loops meet the galactic plane (the troughs).

Loops and Stars

But, Morris says, the Japanese team's report still leaves open some important questions.

One issue is that researchers had expected such loops to contain lighter gases made from individual atoms, such as hydrogen.

Instead the Japanese researchers report that the observed loops carry heavier gases made of molecules, such as carbon monoxide.

"I don't necessarily claim that it's a problem for [the Parker instability] theory," Morris said. "It just doesn't match our expectations."

Another sticking point, he adds, is that researchers must now confirm that the loops seen by the Japanese team are indeed caused by magnetic fields—a difficult task given that magnetism can't been visually observed.

Ultimately, understanding such loops could be significant, Morris says, because the features may play a key role in star formation in the Milky Way and other galaxies (related image: star formation in the Orion Nebula).

Gravitational forces whirl most of the gas, dust, stars, and other matter in our galaxy into a flat, rotating disk centered on a supermassive black hole.

But the two giant loops observed by the Japanese team float above the main surface, or plane, of our Frisbee-shaped galaxy, buoyed—in theory—by magnetic fields.

These loops reconnect to the main plane of our galaxy at regions known as foot-points, areas where molecular gas naturally concentrates.

Such pools of dense molecular gas are what create the right conditions for star formation.

Morris said that "the concentration of gas at the footprints of the loops has been considered for a long time as a plausible mechanism for inducing star formation."

The astrophysicist noted that scientists could easily confirm this theory using NASA's orbiting Spitzer Space Telescope to "see if there's a lot of star formation activity" in these zones.

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