APOD: Edge-On NGC 891 (2017 Jan 12)

[neufer]

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?

---

<<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]

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

[starsurfer]

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

A beautiful weirdo?

[Chris Peterson]

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.

The papers about this make interesting reading. I tracked them down because I was curious how you determine the rotation direction of a face-on galaxy. The answer is, you don't. Luckily, however, truly face-on galaxies are rare, and this one has an inclination of around 20°, which is enough that by doing a redshift analysis on distributed Ha clouds, the direction of rotation can be inferred.

[isenor]

I think they exist because we are in a galaxy and we exist

[MarkBour]

... The spiral galaxy, NGC4622 (also called backward galaxy) ...

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

Those are just absolutely fascinating ... thanks!

Not too long ago, I was sufficiently unschooled to think that when you saw a spiral galaxy, the curves showed the motion of the dust and stars in the same way as one sees material flowing down a shower drain.

I'd like to learn more about "galactic mechanics", but haven't yet devoted any appreciable amount of time to it. But from my dabbling through APOD, more recently, 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".

I don't know if this "history-trace" idea, a term I just made up, is already an established and popular model, or an already debunked idea, but it would seem to be able to be reconciled with what is seen in NGC4622. In its case, I would think that some massive event caused the whole inner part to reverse its direction. Then, after that point, it would have created inner arms that are wrapped in the opposite direction of the outer arms.

And if this model is correct, then it could be that the outer arms are still rotating in opposition to the rotation of the inner part. If there was enough interaction, perhaps through dark matter, they might reverse direction as well, but I would think that would result in a huge mess, if it were possible.

Even if this idea is correct for some galaxies, 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.

[neufer]

... The spiral galaxy, NGC4622 (also called backward galaxy) ...

... 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.

[rstevenson]

... 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. ...

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.

Think of a tidal bore where the individual molecules of water go sliding through the wave while the wave stands still or even progresses against the flow of water. Stars are like the molecules of water, merrily going on their own way, following their own orbit around the center of mass of the galaxy. As they go around the galaxy they pass into and out of the arms.

Our Sun is currently located in the Orion-Cygnus Arm, a minor off-shoot of the Sagittarius Arm of the Milky Way. There's a good article with illustrations on Universe Today.

Rob

[Ann]

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.

Those are some great videos, Rob.

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".

The way I understand it, astronomers agree that the arms of spiral galaxies typically trail.

But I don't think that galactic bars shrink very often. They may shrink sometimes, for all I know, but I believe that astronomers see far fewer barred galaxies at high redshift than they do in the nearby universe. In other words, galactic bars have become more common over time, so they must grow by some mechanism.

Click to view full size image

NGC 3504. Photo: SDSS/Galaxyforum.

In some cases, you can see that barred galaxies display enhanced star formation at the ends of their bars. I believe that this "bar-end star formation" may increase the length of galactic bars.

Ann

[MarkBour]

Ann and Rob, thanks for both of these answers.
I'll have to read the references given. Fascinating. If the spiral arms are not "material", then they are kind of like iron filings near a magnet -- they are revealing something for us that would otherwise be unseen.

[Ann]

On second thought, I think I have to back down from my claim that star formation at the ends of bars cause the lengthening of bars. Instead, I think Rob is right: Not only does galactic rotation cause "standing waves" and spiral arms, but I think that changes in the rotation pattern are the root cause of bars.

Click to view full size image

Barred galaxy M61. ESA/Hubble and NASA.
Acknowledgements: G. Chapdelaine, L. Limatola, and R. Gendler.

Click to view full size image

Barred spiral galaxy NGC 1300. Image credit:
Pat Knezek (WIYN), NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

Take a look at barred spiral galaxy M61 at left. As far as I know, all barred spiral galaxies have long dust lanes running out from the nuclear region all the way into where the spiral arms take over. I believe that all these barred spirals also have a tiny spiral in their nuclear regions, and you can clearly see this tiny "nuclear spiral" in the picture of M61. Spectacularly barred galaxy NGC 1300 also has very long dust lanes running out from its center, and you can discern a tiny inner spiral. Interestingly, both M61 and NGC 1300 display enhanced star formation at the ends of their bars. But this enhanced star formation at the ends of their bars is almost certainly a consequence of the pileup of gas at the ends of the bars, and not the cause of the lengthening of the bars per se. I think it is possible to think of the long dust lanes as freeways where gas can flow outwards unhindered until it runs into the traffic jam at the ends of the bars, where it piles up and leads to star formation.

And the reason why gas would flow outwards along the long bar dust lanes is, I think, dynamics in the core of the galaxy, including the central black hole. Central black holes are indeed associated with gaseous outflows, as can be seen in this Chandra X-ray portrait of supergiant elliptical galaxy M87 and its enormously massive central black hole.

Ann