Spiral Galaxy Winds Backwards

Spiral Galaxy Winds Backwards
This color image of NGC4622 shows the strong inner counter-clockwise outward winding single arm and the strong outer clockwise outward winding pair of arms.
(Image: © NASA)

Newdiscoveries in a strange spiral galaxy show it has a pair of arms winding backwardcompared to the typical direction for most galaxies.

?While theexistence of a galaxy with a pair of ?backward? arms may seem like aninconvenient truth to many, our latest analysis indicates it is, nonetheless, areality,? said Gene Byrd, a University of Alabama astronomer.

Most spiralarms observed so far tend to trail in the wake of their galaxy's spin,meaning they wind in the direction opposite the rotation. The strange galaxy,known as NGC4622, lies 200 million light years away and has a large outer armpair that winds clockwise.

Byrd andhis colleagues analyzed a 2001 Hubble Space Telescope image of the galaxy andfound a previously hidden inner pair of arms winding counter-clockwise.Whichever way the galaxy happens to rotate, one pair of arms ends up turning inthe unusual direction.

?Contraryto conventional wisdom, with both an inner counter-clockwise pair and an outerclockwise pair of spiral arms, NGC4622 must have a pair of leading arms,? Byrdsaid. ?With two pairs of arms winding in opposite directions, one pair mustlead and one pair must trail."

The teamalso found a single outer clockwise arm and a single inner counterclockwise arm,which again points to the galaxy's strange characteristic.

Byrd andhis colleagues first published the idea about the backward arms in 2002, butmet with skepticism from astronomers who thought that the galaxy's slight tiltand clumpy dust clouds could be misleading.

This timethe team used a new Fourier component method that takes advantage of the tiltto analyze the galaxy and ignores the effects of dust.

The morecomplicated analysis of the image revealed that the strong outer clockwise pairof arms winds in the same direction as the galaxy's spin, making it the unusualleading arm pair. The full results appear in the January issue of AstronomicalJournal.

Questionsstill remain about what led to the galaxy's strange behavior. The Hubble imagerevealed a dark dust lane in the galaxy center, suggesting NGC4622 may have devoureda smaller galaxy.

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