June 2, 2006 — Our galaxy is a much wilder looking tentacled beast than suspected, say astronomers who have used a new technique to map the Milky Way.
Using data from a painstaking galaxy-wide survey of hydrogen gas clouds throughout the Milky Way, radio astronomers found that instead of being a clean, symmetrical spiral, our home galaxy has extra long arms sprouting out on one side and is warped and armless on the other.
The odd layout could be the result of colossal cannibalistic galactic collisions or intergalactic "tides."
Astronomer Leo Blitz of the University of California at Berkeley said the most surprising thing about the spiral arms on our half of the Milky Way "is that we could see spiral structure all the way to the edge of the (galactic) disk."
The long, elegant, spiral arms are visible, despite a galaxy full of dust, because hydrogen gas emits radio waves that penetrate the dust.
The distances to the various arms could be worked out by the Doppler shift in their radio waves — similar, in principle, to how the tone of a train’s horn sounds higher-pitched when it’s approaching than when it’s receding.
Blitz co-authored a paper on the newfound shape of the Milky Way with graduate student Evan Levine in the June 1 edition of Science Express.
In contrast to the near side of the Milky Way, the far side appears less orderly. Instead of having clearly defined curved arms there’s a messy bulging area, explains Levine.
"It’s bent like a vinyl record in the sun," Levine told Discovery News. "That would show up in a map like this." As for what causes the weird warping, it could be the result of galactic collisions.
"This lack of symmetry often occurs as galaxies eat little galaxies," Blitz told Discovery News.
Another possibility is that the bulge is caused by the gravitational tug of the Milky Way’s small satellite galaxies. The most obvious of these satellite galaxies are the Magellanic Clouds, which are visible to the naked eye from Earth’s southern hemisphere.
"The Magellanic Clouds are very prominent, like the tides of the Earth," said astrophysicist Frank Shu of the University of California at San Diego. Earth’s oceans bulge with tides because of the external gravitational tug of the Moon. Likewise, the Magellanic Clouds may cause the bulge in the far side of the Milky Way, he explained.
More details about the Milky Way’s structure are likely to be forthcoming, said Levine. Their new map has a few blind spots caused by the motion of our own solar system inside the Milky Way. These blind spots show up on their map as an empty circle at the center of the galaxy and as two blank wedges on opposite sides of our solar system.
It’s possible that more advanced analyses of the data will glean meaningful information from the hydrogen emissions of those areas as well, said Levine.
For his part, Blitz is pleased to be able to see so much of the galaxy after many years of trying. He recalls that in 1982 he published another paper in Science describing only a part of a single spiral arm.
"It’s like trying to figure out the shape of a forest when you’re sitting in the middle of it," said Blitz.
