Starship Asterisk*

by **Ann** » Sat Jan 28, 2017 5:20 am

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

by **MarkBour** » Fri Jan 27, 2017 9:52 pm

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.

by **Ann** » Fri Jan 27, 2017 7:17 pm

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.

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

by **rstevenson** » Fri Jan 27, 2017 5:54 pm

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. ...

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

by **neufer** » Fri Jan 27, 2017 5:51 pm

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.

by **MarkBour** » Fri Jan 27, 2017 5:26 pm

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

Ann wrote:... 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.

by **isenor** » Sat Jan 21, 2017 8:41 am

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

by **Chris Peterson** » Thu Jan 19, 2017 3:01 pm

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.

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.

by **starsurfer** » Thu Jan 19, 2017 2:32 pm

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

A beautiful weirdo?

by **Ann** » Thu Jan 19, 2017 5:57 am

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

by **neufer** » Thu Jan 19, 2017 3:26 am

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:
<<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.>>

by **MarkBour** » Thu Jan 19, 2017 1:46 am

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

: M101.jpg (27.21 KiB) Viewed 7237 times

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?

by **Ann** » Fri Jan 13, 2017 2:49 am

DL MARTIN wrote:The description of this galaxy as being a distance away is an astronomical (pun intended)l conceit in that it can only be qualified as an entity visualized as it existed in the past. To suggest otherwise is to award temporal context equivalent to evaluating an archeological specimen as currently viable. This deludes the public into believing that galaxies, for example, are just over there when we have no evidence that they still even exist.

You can be sure that NGC 891 still exists.

Galaxies, unlike individual massive stars, can't "explode as supernovas" and disappear. They can, indeed, merge with other galaxies and be incorporated into the resulting, larger galaxy. It is also possible that some lightweight, fluffy dwarf galaxies might gradually lose mass until they become to "weak" to hold on to their individual stars, so that they disperse and disappear that way.

But NGC 981 has a bright yellow population and is therefore relatively massive. It certainly has enough self-gravity to keep itself together. NGC 891 is also rather isolated in space. It doesn't have any large neighbors at all.

Our own galaxy, the Milky Way, does indeed have a quite massive neighbor galaxy, the Andromeda galaxy. The Andromeda galaxy is only about 2 million light-years away, and the Milky Way is indeed destined to merge with Andromeda. But in spite of the fact that these two galaxies are so close, the collision isn't supposed to happen for a few billion years yet.

We see NGC 891 as it was only 30 million years ago. That's nothing in the life of a non-interacting galaxy. You can be sure that NGC 891 still looks much the same way "today" as it did 30 million years ago, when the photons that carry the information necessary for earthly astronomers to make pictures of it here on Earth, left the galaxy.

Ann

by **Ann** » Fri Jan 13, 2017 12:17 am

MarkBour wrote:
Ann wrote: ... but note that half the dust lane is blue from star formation. ...
Interesting point. Would you say that the most active star formation visible in this galaxy is occurring in outer arms? I see what you are saying about the blue all along the left side, and I think I see a smaller, but sizable, active region at the far right. Imagining that this is a spiral galaxy with some arms, I'm guessing that the left half of what we can see is an arm, much like our own arm of the Milky Way.

I'm wondering what rotation this galaxy has (e.g. is the left half approaching us and the right half retreating, or the other way around). Are there visual characteristics of spiral arms that are typical that might give a clue of that? I don't know if a doppler shift difference would be sufficiently large to be used to answer that.

Mark, I'm still ashamed for missing the fact that the APOD was plagiarized, but I'll try to answer your question as best I can.

: Click to view full size image

NGC 1532. Copyright:
Volker Wendel and Bernd Flach-Wilken.

Star formation often happens asymmetrically. NGC 1532 is a good example. You can see that a part of one arm, at upper right, is extremely active in star formation, as is obvious from the pink color of its bright emission nebulas. The rest of the galaxy isn't forming very many new stars.

There are some important differences between NGC 891 and NGC 1532, most notably that NGC 1532 is interacting with another galaxy and is tidally disturbed by this interaction. NGC 891, by contrast, looks very serene, at least when it comes to its overall shape.

But I think that NGC 891 and NGC 1532 are similar in that they form stars asymmetrically, so that star formation is more active in some parts of their arms than in others. So I don't think that the difference in color of the dust lane of NGC 891 has anything to do with Doppler shift. But I agree that it should certainly be possible to use Doppler shift measurements to find out how NGC 891 is rotating. Probably such measurements already exist, but unfortunately I don't know the results of them.

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

by **Ann** » Thu Jan 12, 2017 11:16 pm

Adam, I'm sorry for not spotting the plagiarism!

Ann

by **BobGillette** » Thu Jan 12, 2017 9:57 pm

Stupid me. I glanced at the image this morning and said, hm, looks like an Adam Block. Then I went to the website of the claimant and thought, hm. How'd he do that with a C11?

Sure enough, it was an Adam Block.

Never tumbled to the diffraction spikes.

Bob Gillette

by **ignacio_db** » Thu Jan 12, 2017 7:14 pm

I suspected this, the angles of the spikes were identical, as well as the whole look of the image. So lame...

Oh well, I guess we still need an 0.88m scope to get there.

by **RJN** » Thu Jan 12, 2017 6:53 pm

This APOD has now been updated. Please see the Editor's Note now appended to the APOD text. The note states "The NGC 891 image used in today's APOD posting has been replaced and the credit corrected to indicate the author of the original work." Thanks to everyone who alerted us about the situation. We apologize for the inconvenience. - RJN

by **geckzilla** » Thu Jan 12, 2017 6:52 pm

Today's APOD image was plagiarized. It's known. APOD editor is working on it. People do stupid things for the rat race. We move on once again.

by **csp** » Thu Jan 12, 2017 6:15 pm

Chris Peterson wrote: Nevertheless, they look a lot more like synthesized spikes than real ones, given the total lack of dispersion.

Actually even the reflections around the stars are completely identical on the 2 pictures. The other one is I think a 24 inch RC.
I have a Celestron EdgeHD as well, I know how stars looks like normally. Completely different than here. This must be an enhanced SCT or so...

by **MarkBour** » Thu Jan 12, 2017 5:58 pm

Ann wrote: ... but note that half the dust lane is blue from star formation. ...

Interesting point. Would you say that the most active star formation visible in this galaxy is occurring in outer arms? I see what you are saying about the blue all along the left side, and I think I see a smaller, but sizable, active region at the far right. Imagining that this is a spiral galaxy with some arms, I'm guessing that the left half of what we can see is an arm, much like our own arm of the Milky Way.

I'm wondering what rotation this galaxy has (e.g. is the left half approaching us and the right half retreating, or the other way around). Are there visual characteristics of spiral arms that are typical that might give a clue of that? I don't know if a doppler shift difference would be sufficiently large to be used to answer that.

by **Chris Peterson** » Thu Jan 12, 2017 5:50 pm

csp wrote:
They're not diffraction spikes.
I still think they are "real" diffrac. spikes. Check the aligned images, the other one is from a completely different source, I guess an RC. Spikes cannot be reproduced artificially so perfect.

http://g.recordit.co/l8WMbobs6s.gif

I agree (based on other evidence) that the diffraction spikes are present in at least one channel of the source data. Nevertheless, they look a lot more like synthesized spikes than real ones, given the total lack of dispersion.

by **csp** » Thu Jan 12, 2017 5:47 pm

They're not diffraction spikes.

I still think they are "real" diffrac. spikes. Check the aligned images, the other one is from a completely different source, I guess an RC. Spikes cannot be reproduced artificially so perfect.

http://g.recordit.co/l8WMbobs6s.gif

by **Visual_Astronomer** » Thu Jan 12, 2017 5:09 pm

They don't look like real diffraction spikes to me, either, but it is still a pretty picture.

I've looked at NGC891 many times, it is spectacular through the eyepiece.

by **Chris Peterson** » Thu Jan 12, 2017 3:20 pm

csp wrote:Incredible details, nice pict. How it is possible to have diffraction spikes with a SCT? (It was taken with a C11@f10, f6,3 according to the spec of the creator)

They're not diffraction spikes. Look at them closely and they look nothing like diffraction spikes. They're some kind of effort at an aesthetic effect created by post processing. (People who like the look of diffraction spikes but use SCTs or refractors sometimes create them by placing crosshairs across their apertures... but in this case, the uniform hue of the spikes reveals they were created after the image was made.)

Starship Asterisk*

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

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