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.
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