Spiral Galaxy Winds Backwards

New discoveries in a strange spiral galaxy show it has a pair of arms winding backward compared to the typical direction for most galaxies.

"While the existence of a galaxy with a pair of 'backward' arms may seem like an inconvenient truth to many, our latest analysis indicates it is, nonetheless, a reality," said Gene Byrd, a University of Alabama astronomer.

Most spiral arms 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 arm pair that winds clockwise.

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

"Contrary to conventional wisdom, with both an inner counter-clockwise pair and an outer clockwise pair of spiral arms, NGC4622 must have a pair of leading arms," Byrd said. "With two pairs of arms winding in opposite directions, one pair must lead and one pair must trail."

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

Byrd and his colleagues first published the idea about the backward arms in 2002, but met with skepticism from astronomers who thought that the galaxy's slight tilt and clumpy dust clouds could be misleading.

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

The more complicated analysis of the image revealed that the strong outer clockwise pair of arms winds in the same direction as the galaxy's spin, making it the unusual leading arm pair. The full results appear in the January issue of Astronomical Journal.

Questions still remain about what led to the galaxy's strange behavior. The Hubble image revealed a dark dust lane in the galaxy center, suggesting NGC4622 may have devoured a smaller galaxy.

Jeremy Hsu
Jeremy has written for publications such as Popular Science, Scientific American Mind and Reader's Digest Asia. He obtained his masters degree in science journalism from New York University, and completed his undergraduate education in the history and sociology of science at the University of Pennsylvania.