NGC 4622: An inconvenient galaxy
Published on Jan 9, 2008 at 12:07 am.
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Filed under conference blogging, galaxies.
I got the title for this posting from a poster presented by Gene Byrd, et al, at the AAS Meeting. A few years ago, he was studying the image seen above for the galaxy NGC 4622. A quick glance at the galaxy shows that it is a beautiful spiral galaxy. However, if you look at it carefully, you see something odd. There are two different sets of spiral arms. That, in itself, would be a little unusual, but not really noteworthy, except that these two sets of spiral arms wind in opposite directions! As far as I know, this is the only case of that in a spiral galaxy. The inner spiral arms wind clockwise, and the outer spiral arms wind counterclockwise. Now, that would be downright bizarre. But, the strangeness doesn’t stop there. Earlier studies indicated that the outer spiral arms are reversed from the direction of galactic rotation.
Now, let me explain. Spiral galaxies nearly universally rotate as the look, with the spiral arms trailing the direction of rotation. So, the prominent spiral arms in this image would tend to indicate that the galaxy were rotating counterclockwise. Clearly, the inner spiral arms tell a different story, though. So, a first guess might be that this galaxy is the result of a merger, in which the inner part of the galaxy rotates one way and the outer portion rotates in another direction. But, the outer spiral arms of this galaxy seem to go backwards. In other words, the spiral arms lead. But, this is a very unsatisfying finding, and many astronomers doubted the results.
Now, Byrd, et al, present new Fourier analysis showing that the disk does rotate clockwise, meaning that the outer spiral arms are, indeed backwards! This is an independent method of analysis showing the rotation direction of the galaxy, and confirming earlier findings that the outer spiral arms are backwards. In other words, the outer spiral arms are leading the rotation, not trailing it. What makes this whole finding even tougher to explain, though, is that there appears to be no disruption of the disk between the inner and outer regions, other than that the spiral arms go in the other direction. Further, they found that there are fainter spiral arms embedded in the disk in both inner and outer portions of the disk that seem to wind the same way. So, that means that there is definitely something weird about this galaxy. It is a conundrum to try to explain. You can find more about their work in a document posted on Dr. Byrd’s site. And, the complete research paper is published in the January, 2008, issue of The Astronomical Journal. Clearly, this galaxy needs more study. It still is not clear why the spiral arms are behaving as they are.
-Astroprof
Image courtesy of NASA, G. Byrd
