Starship Asterisk*

MarkBour wrote:

Ann wrote:
Finally, few edge-on spiral galaxies provide us with visual clues about which way they are rotating. NGC 7331, which isn't really edge-on, is one exception to the rule. You can clearly see, just by looking at it, which way it is rotating.

Ann -- Just to double-check ... I assume you're saying that when one sees a grand spiral galaxy with arms, such as at the right, we know without measuring that it must be rotating counter-clockwise (from our view). Nobody has ever come across a spiral galaxy and found out it is not rotating, or even rotating "against" the apparent sweep of the spiral arms, right?

https://en.wikipedia.org/wiki/NGC_4622 wrote:

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<<The spiral galaxy, NGC4622 (also called backward galaxy), lies approx. 200 million light years away in the constellation Centaurus. NGC 4622 is a face-on unbarred spiral galaxy with a very prominent ring structure located 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.>>

Ann wrote:Art made a good point. NGC 4622 is a weirdo. I think the "Black-Eye Galaxy", M64 (I think it's that one - don't have time to check it up) also has an inner and an outer disk rotating in different directions.

But in the overwhelming number of cases, galaxies rotate with their arms "trailing behind them".

Ann

Ann wrote:Art made a good point. NGC 4622 is a weirdo. I think the "Black-Eye Galaxy", M64 (I think it's that one - don't have time to check it up) also has an inner and an outer disk rotating in different directions.

neufer wrote:... The spiral galaxy, NGC4622 (also called backward galaxy) ...

Ann wrote:... the "Black-Eye Galaxy", M64 ... Ann

MarkBour wrote:

neufer wrote:
... The spiral galaxy, NGC4622 (also called backward galaxy) ...

Ann wrote:
... the "Black-Eye Galaxy", M64 ... Ann

I'm sure there would be others that would form in different manners altogether,
so alternatively, perhaps that would be the case for these two weirdos.

MarkBour wrote:... I have come to the thinking that when you see a spiral galaxy, the arms you see may often be a "history trace" of a rotating bar that kept losing its end material and shrinking. ...

rstevenson wrote: I think the currently accepted theory is that the arms of a spiral galaxy are standing waves. This is the Spiral Density Wave Theory. On that Wikipedia page there is a terrific video showing just how stars move in their orbits while the spirals themselves don't move. Also, the diagram just above that video clearly shows how the elliptical orbits of all stars create the spiral illusion.

MarkBour wrote:
I have come to the thinking that when you see a spiral galaxy, the arms you see may often be a "history trace" of a rotating bar that kept losing its end material and shrinking. If that were true, then in most cases, the whole thing would be rotating in the same direction as one would move by following the spirals inward. In other words, the arms would "trail".