direction of galaxy rotation

[dmzer5]

I always enjoy looking at -- and am awed by -- the great views of galaxies, especially the spiral galaxies. But I have a query: what is their direction of rotation? Do the arms point ahead or behind the rotation?
--david_maranz@sil.org

[neufer]

I always enjoy looking at -- and am awed by -- the great views of galaxies, especially the spiral galaxies.
But I have a query: what is their direction of rotation? Do the arms point ahead or behind the rotation?

[b][color=#0000FF]Spiral arms are essentially fixed standing waves and stars orbit into the concave side of these arms and come out the convex side of these arms.[/color][/b]

---

http://apod.nasa.gov/apod/ap020125.html


<<NGC 4622 is unique in that it is the only known system of this type to have outer arms that point toward the direction of the galaxy's rotation (clockwise), while the inner arms point in the opposite direction. The unique rotational configuration of the arms may be the result of a past minor merger or a mild tidal encounter with a smaller companion galaxy. Possible evidence for a minor merger is a short, central dust lane, although this is circumstantial.>>

<<The spiral galaxy, NGC4622 (also called backward galaxy), lies 111 million light years away in the constellation Centaurus. NGC 4622 is an example of a galaxy with leading spiral arms. In spiral galaxies, spiral arms were thought to trail; the tips of the spiral arms winding away from the center of the galaxy in the direction of the disks orbital rotation. In NGC4622, however, the outer arms are leading spiral arms; the tips of the spiral arms point towards the direction of disk rotation. This may be the result of a gravitational interaction between NGC 4622 and another galaxy or the result of a merger between NGC 4622 and a smaller object .

NGC 4622 also has a single inner trailing spiral arm. Although it was originally suspected that the inner spiral arm was a leading arm, the observations that established that the outer arms were leading also established that the inner arm was trailing.

These results were met with skepticism in part because they contradicted conventional wisdom with one quote being “so you’re the backward astronomers who found the backward galaxy.” The fact that a pair of arms could lead was not easy to accept. Astronomical objections centered on the fact that dust reddening and cloud silhouettes were used to determine that the outer arms led. The galaxy disk is tilted only 19 degrees from face-on making near to far-side effects of dust hard to discern and because clumpy dust clouds might be concentrated on one side of the disk, creating misleading results.

In response, the “backward astronomers” determined NGC4622’s spiral arm sense with a method independent of the previous work. The new Fourier component method is actually assisted by the small tilt, and dust reddening and cloud silhouettes are not used in the latest analysis. The Fourier component method reveals two new weak arms in the inner disk winding opposite the outer strong clockwise pair. Thus the galaxy MUST have a pair of arms winding in the opposite direction from most galaxies. Analysis of a color-age star formation angle sequence of the Fourier components establishes that the strong outer pair is the leading pair.

While the presence of backward arms in a galaxy may seem like an inconvenient truth to many, two independent methods now indicate that NGC4622’s arms do indeed behave in a very unusual fashion, with the outer arms winding outward in the same direction the disk turns.>>

[bystander]

http://asterisk.apod.com/viewtopic.php?f=8&t=14572

[dougettinger]

I always enjoy looking at -- and am awed by -- the great views of galaxies, especially the spiral galaxies.
But I have a query: what is their direction of rotation? Do the arms point ahead or behind the rotation?

[b][color=#0000FF]Spiral arms are essentially fixed standing waves and stars orbit into the concave side of these arms and come out the convex side of these arms.[/color][/b]

---

http://apod.nasa.gov/apod/ap020125.html


<<NGC 4622 is unique in that it is the only known system of this type to have outer arms that point toward the direction of the galaxy's rotation (clockwise), while the inner arms point in the opposite direction. The unique rotational configuration of the arms may be the result of a past minor merger or a mild tidal encounter with a smaller companion galaxy. Possible evidence for a minor merger is a short, central dust lane, although this is circumstantial.>>

<<The spiral galaxy, NGC4622 (also called backward galaxy), lies 111 million light years away in the constellation Centaurus. NGC 4622 is an example of a galaxy with leading spiral arms. In spiral galaxies, spiral arms were thought to trail; the tips of the spiral arms winding away from the center of the galaxy in the direction of the disks orbital rotation. In NGC4622, however, the outer arms are leading spiral arms; the tips of the spiral arms point towards the direction of disk rotation. This may be the result of a gravitational interaction between NGC 4622 and another galaxy or the result of a merger between NGC 4622 and a smaller object .

NGC 4622 also has a single inner trailing spiral arm. Although it was originally suspected that the inner spiral arm was a leading arm, the observations that established that the outer arms were leading also established that the inner arm was trailing.

These results were met with skepticism in part because they contradicted conventional wisdom with one quote being “so you’re the backward astronomers who found the backward galaxy.” The fact that a pair of arms could lead was not easy to accept. Astronomical objections centered on the fact that dust reddening and cloud silhouettes were used to determine that the outer arms led. The galaxy disk is tilted only 19 degrees from face-on making near to far-side effects of dust hard to discern and because clumpy dust clouds might be concentrated on one side of the disk, creating misleading results.

In response, the “backward astronomers” determined NGC4622’s spiral arm sense with a method independent of the previous work. The new Fourier component method is actually assisted by the small tilt, and dust reddening and cloud silhouettes are not used in the latest analysis. The Fourier component method reveals two new weak arms in the inner disk winding opposite the outer strong clockwise pair. Thus the galaxy MUST have a pair of arms winding in the opposite direction from most galaxies. Analysis of a color-age star formation angle sequence of the Fourier components establishes that the strong outer pair is the leading pair.

While the presence of backward arms in a galaxy may seem like an inconvenient truth to many, two independent methods now indicate that NGC4622’s arms do indeed behave in a very unusual fashion, with the outer arms winding outward in the same direction the disk turns.>>

I do believe that NGC 4622 is "half-backward". A diagram of the MilkyWay shows the Sun orbiting in the opposite direction from what is stated as a natural spiral galaxy: The tips of the spiral arms point in the direction of the rotation of the galaxy's disk. Can anyone help me with my confusion ? Thanks.

Doug Ettinger
Pittsburgh, PA