spiral arms in dark-matter galaxies

[Procyan]

This question keeps going around in my mind. It bugs me enough to trouble ya'll with it

I understand that there was mystery about missing matter. The rate of rotation (RPA, a for aeons ?) of distal stars was the same as those proximal to the galactic center. And I understand sorta that that could only be true if there was a uniform distribution of mass across the disc, unlike a solar system where Kepler rules. And I accept with enthusiasm the discovery of dark matter.

But why then do the arms of many galaxies appear curved? The spiral shape strongly implies, you know, differential rates. The further out the smaller the arc swept out per time bleem. :confused:. I'm wondering if someone can solve this little puzzle for me? To me there is contradiction because the discs should look uniform without spirals, Nez Pierce?

[WayneFrancis]

Hmm good question. I'd also ask why the bars are the way they are too. I remember astronomy cast touching on the subject....time for me to do some reading.

[Hornblower]

This question keeps going around in my mind. It bugs me enough to trouble ya'll with it

I understand that there was mystery about missing matter. The rate of rotation (RPA, a for aeons ?) of distal stars was the same as those proximal to the galactic center. And I understand sorta that that could only be true if there was a uniform distribution of mass across the disc, unlike a solar system where Kepler rules. And I accept with enthusiasm the discovery of dark matter.

But why then do the arms of many galaxies appear curved? The spiral shape strongly implies, you know, differential rates. The further out the smaller the arc swept out per time bleem. :confused:. I'm wondering if someone can solve this little puzzle for me? To me there is contradiction because the discs should look uniform without spirals, Nez Pierce?
Suppose, in a thought experiment, we create a pattern of bright stars that form an arm sticking straight out, and have all of them in circular orbits going at the same speed. Since the outer ones have bigger circles to cover, the arm will become curved, with the outer parts trailing.

If the motions instead had followed a Keplerian pattern, with the outer parts moving more slowly, the curvature would simply be sharper.

[Cougar]

The rate of rotation... of distal stars was the same as those proximal to the galactic center.
Approximately. And the stars very near the center do move considerably faster. But at some point, the rotation curve flattens out, and Kepler scratches his head in his grave.


And I understand sorta that that could only be true if there was a uniform distribution of mass across the disc...
A key realization. A huge black hole mass at the center would not have this effect. It's apparently a spherical uniform distribution.


But why then do the arms of many galaxies appear curved? The spiral shape strongly implies, you know, differential rates.

Well, oddly, the arms are not defined by a fixed set of orbiting stars. I guess there are a couple theories about this, but the one I've heard most is that the arms are density waves (http://en.wikipedia.org/wiki/Density_wave_theory) within the galaxy, and those density waves move slower than the orbiting stars.

Plus, I do not imagine that spiral galactic rotation curves are exactly flat. The outer stars orbit somewhat slower than the inner stars -- but not nearly slow enough to satisfy keplerian formulae. Hence the need to hypothesize dark matter.

[mugaliens]

Well, oddly, the arms are not defined by a fixed set of orbiting stars. I guess there are a couple theories about this, but the one I've heard most is that the arms are density waves (http://en.wikipedia.org/wiki/Density_wave_theory) within the galaxy, and those density waves move slower than the orbiting stars.

This is a very good way of explaining it, Cougar. I've always likened it to the war cars slow down and bunch up when there's been an accident on the side of the road. The cars (stars) continue to move through the congestion (spiral arm), which remains at one point. Then, long after the accident has been cleared, they continue to do so, simply because everyone else is slowing down.

Of course the physics behind each are totally different, and the stars themselves don't actually slow down to create the congestion... But the effect is similar and serves to illustrate the point.

Another key point is the rotation rate of the stars within the corotation radius and beyond, as well as the spiral effect's presense only within the inner and outer Linblad resonance radii. Cougars links explains this well.

The physics of the phenomenon brings to question, however, the causal nature of spiral-arm and other galaxies.

[Procyan]

Suppose, in a thought experiment, we create a pattern of bright stars that form an arm sticking straight out, and have all of them in circular orbits going at the same speed. Since the outer ones have bigger circles to cover, the arm will become curved, with the outer parts trailing.

If the motions instead had followed a Keplerian pattern, with the outer parts moving more slowly, the curvature would simply be sharper.

Yep, got it. Equal rates do not imply equal RPM. Once again I blunder.

Thank you

[Procyan]

Oh my goodness! I've read the wiki page. Not dreamt of in my philosophy. I see this is probably well beyond me. My original question is now replaced by another, i hope not to sound stupid but must ask:

"Lin and Shu proposed in 1964 that the arms were not material in nature, but instead made up of areas of greater density, similar to a traffic jam on a highway.[4] The cars move through the traffic jam: the density of cars increases in the middle of it. The traffic jam itself, however, does not move (or not a great deal, in comparison to the cars). In the galaxy, stars, gas, dust, and other components move through the density waves, are compressed, and then move out of them."

If the wave is not Material then what is it? Gravity field?

Of course the cars are stars so that is surely material. But perhaps that is secondary to the point that the arm, although it contains more material, is region where clumping occurs; stuff just collects there? How can they become "compressed" without slowing down?....Nope, I don't get that at all.

You say (I assume one or more of you wrote the page) that the cars move through the traffic jam, but in reality the stars do not slow down.

However, I do take the point that spiral galactic rotation curves are not exactly flat and that multiple forces are at play that contribute to arm conformations.

Thanks very much to all, I will try thinking and reading more. More comment welcomed!

[01101001]

If the wave is not Material then what is it?

Some time in grade school, I began thinking of them as density waves not as solid, or even unsolid, unified groups objects.

In some ways, they are much like ocean waves, which are made of water molecules, but a different group of water molecules at each instant, constantly changing.

So, why do you propose the stars would not speed up and slow down to participate in the density wave, in reaction to the groups ever-changing gravity? While long-term their average speeds may dominate their own orbital motion, minor changes in the short run could produce the illusion of moving arm structures.

[John Mendenhall]

"Lin and Shu proposed in 1964 that the arms were not material in nature, but instead made up of areas of greater density, similar to a traffic jam on a highway.[4] The cars move through the traffic jam: the density of cars increases in the middle of it. The traffic jam itself, however, does not move (or not a great deal, in comparison to the cars). In the galaxy, stars, gas, dust, and other components move through the density waves, are compressed, and then move out of them."



I heard Shu give a public lecture on his theories in the mid '70s. He was great.

[Procyan]

Now I see. Actual waves. Hence the term Acoustic. Absolutely incredible.

Where could they originate? Could these acoustic galactic waves date to some primordial violence? The wiki page sounds like it may simply be an emergent feature of large rotating mass (my inference). Do the computers show that?

I've wondered if the sun's cycle is an "echo" of its original collapse and ignition. Is it still rebouncing from that every 11/22 years? Do all/most stars oscillate? Could they sync and somehow produce the galactic density wave?

Wacky theories, the best thing about being a dilettante. (Downside...ignorance!)

[Procyan]

Hmm good question. I'd also ask why the bars are the way they are too. I remember astronomy cast touching on the subject....time for me to do some reading.

In the light of the other replies, your question becomes even more interesting. Is it correct to ask: Why are the co-rotational frames of bars larger than spirals?

Because they are are akin to sound waves, they propagate independently of the motion of the stars. I'd bet some on this list are studying the relationship between density waves and dark matter at this moment. Fantastic! What might we learn from this? Does dark matter have a characteristic resonant frequency? Has Ferris or Kaku written about these phenomena? I would love to read more for understanding.

[mugaliens]

It's similar to what happens when you put a bunch of kids on one of those inflatable jumping platforms, only it's got a leak... The kids tend to clump together, but that doesn't last long, as they'll go blasting apart again soon enough.

As an individual star enters a region of a greater density of stars, it accelerates. Most simply pass through, only slightly altered in their trajectories. Some get flung about, thereby becoming part of, and contributing to the arm's density. Occasionally, a star that's been a part of the arm for a while will get flung out by a star that's just entered.

And so, life goes on.

[Procyan]

So, why do you propose the stars would not speed up and slow down to participate in the density wave, in reaction to the groups ever-changing gravity? While long-term their average speeds may dominate their own orbital motion, minor changes in the short run could produce the illusion of moving arm structures.

My misconception was that if the waves were not "material" then there would be no participation by individual stars. I was hung up on material vs. non-material. Now I understand the acoustic nature of the wave a little better, thanks! However, still not quite there. If the wave propagates through a disc that is predominately dark matter, then the motion of the stars is not powerfullly dictated by "star-star" interactions, rather they ride the wave as it passes through the dark matter. Like floaty ducks bunching up at the crest of a water wave, right?

Perhaps I'm going too far with the sound analog? A sound wave is, I think, a shock wave. But the density wave may be a gravitational phenomena.

[Procyan]

It's similar to what happens when you put a bunch of kids on one of those inflatable jumping platforms, only it's got a leak... The kids tend to clump together, but that doesn't last long, as they'll go blasting apart again soon enough.

As an individual star enters a region of a greater density of stars, it accelerates. Most simply pass through, only slightly altered in their trajectories. Some get flung about, thereby becoming part of, and contributing to the arm's density. Occasionally, a star that's been a part of the arm for a while will get flung out by a star that's just entered.

And so, life goes on.

Sorry, I was writing during your post...And I think it is important...does this mean star to star interactions can be important ...er..microscopically, but on the large scale the overall wave conforms to a statistical gravitational model dominated by the sum of the masses involved?

BTW/ Kids terrify me on those things!