Galaxy’s Arms are Rotating Backwards

by Fraser Cain on January 10, 2008

NGC 4622. Image credit: Hubble
As galaxies rotate, their spiral arms usually sweep back, trailing behind the rotation of the galaxy. But astronomers have found a galaxy that defies this convention, with its arms opening outward in the same direction as the rotation of the galaxy’s disk.

The galaxy, known as NGC 4622, lies 200 million light years away in the constellation Centaurus. A team of American astronomers analyzed images of the galaxy, and discovered that it has a previously hidden inner counter clockwise pair of spiral arms.

“Contrary to conventional wisdom, with both an inner counter-clockwise pair and an outer clockwise pair of spiral arms, NGC 4622 must have a pair of leading arms,” said Dr. Gene Byrd from the University of Alabama. “With two pairs of arms winding in opposite directions, one pair must lead and one pair must trail. Which way is which depends on the disk’s rotation. Alternatively, the inner counter clockwise pair must be the leading pair if the disk turns counter clockwise.”

This isn’t the first time the team announced their findings that NGC 4622 had a leading pair of spiral arms. Other astronomers were skeptical of the result, since the galaxy disk is only tilted 19 degrees from face-on, and clumpy clouds of dust could confuse the results.

The researchers came back and used two different independent techniques to verify the direction the arms are spinning.

Further observations are coming, since images from the Hubble Space Telescope revealed a dark dust lane in the centre of the galaxy. This suggests that NGC 4622 may have consumed a smaller companion galaxy, and this could help explain where the additional spiral arms came from.

Original Source: University of Alabama News Release

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Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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