Why Do Galaxies Spin and What Drives Their Motion?

[sophiecentaur]

If 2 bodies move toward each other,

What two bodies will do is no basis for predicting how a galaxy can form. If you take just two bodies, in the absence of anything else, then they will be in an orbit with each other (unchanging and lasting for ever) or else they may collide / have collided (if their trajectories coincide exactly). But that is an unreal and theoretical situation. In the presence of many other objects, there will be interactions between all the objects in the region and the result will be that they gather together in a local region.

More collisions / very near misses will occur causing Kinetic Energy to be lost (but not Angular Momentum). This produces clumping of material with higher angular velocity around the same position of the local centre of mass (which will also be moving linearly).

 

[FactChecker]

What two bodies will do is no basis for predicting how a galaxy can form. If you take just two bodies, in the absence of anything else, then they will be in an orbit with each other (unchanging and lasting for ever) or else they may collide / have collided (if their trajectories coincide exactly). But that is an unreal and theoretical situation. In the presence of many other objects, there will be interactions between all the objects in the region and the result will be that they gather together in a local region.

I took his statement as more of a simple statement of why some angular momentum is much more likely than none. I did not take it as trying to explain the entire theory of how galaxies formed.

 

[sophiecentaur]

I did not take it as trying to explain the entire theory of how galaxies formed.

Once you get me started . . . .
I was just repeating the message about angular momentum, basically. And, of course, the likelihood that a massive cloud of 'stuff' could ever have zero angular momentum is pretty damn low. Plus, there's a maximum value of net angular momentum because the outermost parts would have escaped if they were going too fast past the CM (hyperbolic orbit).

 

[snorkack]

To think of it, if an elliptical galaxy possesses a central black hole then the individual stars in the galaxy must possesses angular momentum - otherwise they would fall in on the first orbit.
How precisely are the elliptical galaxies devoid of combined angular momentum?

 

[sophiecentaur]

How precisely are the elliptical galaxies devoid of combined angular momentum?

Why should they be devoid? These so-called spiral arms in our galaxy are not like the spirals you see in water going down a plughole. This link discusses them quite fully. The arms are density waves and stars move in and out of them, not 'orbiting' at the same rate that the arms rotate.

Needless to say, it's a non-intuitive business.

 

[snorkack]

Why should they be devoid? These so-called spiral arms in our galaxy

And our galaxy is a spiral galaxy, in contrast to elliptical galaxies.

 

[sophiecentaur]

And our galaxy is a spiral galaxy, in contrast to elliptical galaxies.

The lack of arms says nothing about the orbits of the stars around a central attractor. An armless galaxy will presumably not have the appropriate density of stars to interact with each other and to have resonances to form waves. The arms are merely 'virtual' patterns of star density they are not spokes on a wheel.

 

[snorkack]

The lack of arms says nothing about the orbits of the stars around a central attractor. An armless galaxy will presumably not have the appropriate density of stars to interact with each other and to have resonances to form waves.

It is the lenticular galaxies which are said to be like spirals, only without arms.

 

[sophiecentaur]

It is the lenticular galaxies which are said to be like spirals, only without arms.

I found this link which discusses spiral, lenticular and elliptical galaxies in terms of their angular momentum and 'bulge factor'. It has references from the 1990s and later. Well done if you can read every word!

Imo, it's important to understand that spiral arms are not like spokes on a wheel and they don't rotate 'in step' with the individual stars' orbits. The stars in other galaxies also orbit at similar rates - they would have to or collapse into the centre PDQ.

 

[snorkack]

I found this link which discusses spiral, lenticular and elliptical galaxies in terms of their angular momentum and 'bulge factor'. It has references from the 1990s and later. Well done if you can read every word!

It is interesting that the elliptical galaxies do seem to rotate, without an obvious difference.

The stars in other galaxies also orbit at similar rates - they would have to or collapse into the centre PDQ.

They have to orbit, but they do not have to orbit at similar rates.
Compare solar system.
The seven planets orbit within 7 degrees of Earth.
But look at comets.
Since there is Sun in the middle, even long period comets must have perihelion over 0,005 AU - otherwise they would collide with Sun on first orbit.
But a body with aphelion of 30,33 AU might have perihelion 29,81 AU - that is Neptune. Or it might have perihelion at 20 AU - and it would not be a comet, because it would not have a coma even at perihelion. Or it could be a periodic comet with perihelion at 1 AU. Or a sungrazer with perihelion at 0,01 AU.
When you look at comets, it turns out that their inclinations are not limited to the 7 degrees of the 7 planets. Notoriously the Halley Comet has inclination of 162 degrees - it orbits in retrograde direction.
Looking at nonperiodic comets, the Hale-Bopp comet notoriously has inclination of 89,4 degrees - nearly polar orbit.
Does Oort cloud rotate? That is, when you make a statistic of long period comets, is the number of retrograde comets within the limits of coincidence equal to that of prograde comets, or is there any perceptibly preferred direction?

Solar neighbourhood has retrograde stars, too: Kapteyn´s star is retrograde, though most stars are prograde.
Are there any galaxies where retrograde stars are as common as prograde, or rather, there is no preferred direction to define "retrograde"?

 

[sophiecentaur]

they do not have to orbit at similar rates.

Their orbit period will relate to the mass distribution of the galaxy and the position of the star in question.

When you look at comets, it turns out that their inclinations are not limited to the 7 degrees of the 7 planets.

Is this surprising ? The Oort Cloud is spherical so a comet could (ignoring effects that I / we don't know about) have any orbital plane. The fact that the Oort Cloud is spherical must be to do with its great distance out from the Sun and planets and a lack of the influence of at disc. Comets originate in the cloud and are ejected from time to time as the interactions with the rest of the cloud produce just the right conditions. We will only see bodies which happen to get close enough but I imagine there could be loads of them with orbits way beyond Pluto and some could even end up in deep(er) space.

Does Oort cloud rotate?

If it didn't, it would fall inwards. Same rules apply with gravity and orbits.

 

[DaveC426913]

If it didn't, it would fall inwards. Same rules apply with gravity and orbits.

No I think snork's clarification is valid:

That is, when you make a statistic of long period comets, is the number of retrograde comets within the limits of coincidence equal to that of prograde comets, or is there any perceptibly preferred direction?

I don't see why the Oort Cloud as a whole has to have a net preferred rotation. I can see why it might, but if it didn't, it wouldn't, as you say, fall inwards.

 

[sophiecentaur]

No I think snork's clarification is valid:

I don't see why the Oort Cloud as a whole has to have a net preferred rotation. I can see why it might, but if it didn't, it wouldn't, as you say, fall inwards.

Perhaps the net polar component of the angular momentum of the cloud is in the same direction as the rest of the SS but could it be that outside influences (from no preferred direction) could have injected am, not about the common axis? The Oort objects are very far away and far apart so some different rules or timescale could apply.
I imagine the close spacing of the inner bodies has caused cancellation (net to zero) of non polar am, limited to the small spread about the ecliptic plane and left us with just the polar component. Whereas the OoC could be so far away that it hasn't happened yet. In any case, is there a lot of actual evidence about density around the OoC? How uniform is it? How much more than a "theoretical concept" (Wiki) is it?

 

[DaveC426913]

Perhaps the net polar component of the angular momentum of the cloud is in the same direction as the rest of the SS but could it be that outside influences (from no preferred direction) could have injected am, not about the common axis?

Why must there be a significant net rotation at all? (I don't mean zero; I mean not statistically significant.)

 

[sophiecentaur]

Why must there be a significant net rotation at all? (I don't mean zero; I mean not statistically significant.)

Good point. I was sort of assuming that the original nebula would have a uniform net AM. But if OoC is far enough out then could its behaviour be isolated from the SS is some way? Obviously there's a strong enough attraction to keep it near the Sun and SS but the large gap between them that the subtle changes in energy that produced the planets and the asteroid belt etc. may be too weak at that great distance. I sort of hinted at this, further up.
Resonsnces within the OoC still throw out comets but not very frequently. Maybe it would take a lot longer than 4Billion years for the OoC to change and be more like the rest of the SS. The actual numbers and statistics must count here.
On the topic of net AM: if the bodies out there were close enough together, would you not expect gradual 'near cancellation' of AM, resulting in slow objects falling in towards the SS - but that would be Comet formation, wouldn't it?

 

[DaveC426913]

I also wonder if the net AM would evolve over time, as a particular group of bodies self-selects for suicidal plunge into the hellish inner system. I'm just not sure which group would get disturbed faster: pro-grade bodies or retrograde bodies.

I guess it also suggests a periodic change due to a negative feedback loop (as more *-grade bodies fall inward, the ratio of pro- to retro- will flip).

 

[sophiecentaur]

I'm just not sure which group would get disturbed faster: pro-grade bodies or retrograde bodies.

Imo, there would not be much to select either group to interact with the SS. Why not relate to to relative numbers of each? That Noddy theory at least agrees with observation.

 

[DaveC426913]

Imo, there would not be much to select either group to interact with the SS.

Well, except that they do, occasionally.

That Noddy theory...

That term is unfamiliar to me. I find references to a real event, but I don't know how it translates into a metaphor.

petty/trivial?

*^{Oh My God. Back in high school I worked at a cinema with a very pretty girl named Renata. My hobby at the time was assigning people nicknames, and I assigned her "Noddy". For the last four decades I had no idea it was a real term with a real meaning.}

 

[sophiecentaur]

That term is unfamiliar to me.

It was my noddy theory - i.e. not particularly good science but sounds ok to me.

Well, except that they do, occasionally.

Exactly. At such a distance - well on the edge of the first order gravitational effects of the SS (only enough to keep the cloud in place but not to form a disc)- the other effects due to the larger planets would be even less but perhaps enough to affect the direction of things observed / select coming out of the Ooc.
This is really too personal BS for PF, I think.

 

[berkeman]

Thread closed temporarily for Moderation...

 

[berkeman]

Thread is reopened after cleanup of a post with misinformation in it.

 

[ohwilleke]

I found this link which discusses spiral, lenticular and elliptical galaxies in terms of their angular momentum and 'bulge factor'. It has references from the 1990s and later. Well done if you can read every word!

Imo, it's important to understand that spiral arms are not like spokes on a wheel and they don't rotate 'in step' with the individual stars' orbits. The stars in other galaxies also orbit at similar rates - they would have to or collapse into the centre PDQ.

Incidentally, while it is very easy to piece together an argument that most galaxies should have angular momentum, the scaling relations discussed in the linked paper, which are very regular, are harder to explain and require a much more involved model than the bare bones ones offered up in this thread.

 

[Mendrys]

This is new and describes other processes that contribute to the spin of a galaxy.

https://phys.org/news/2022-10-flips-galaxies-cosmic-web.html

This is from the article and sums it up a bit:

"Galaxies which are mostly disk, with a low-mass bulge, tend to have their spin axis parallel to the nearest filament. This is because they form mainly from gas falling onto the filament and 'rolling it up.' Galaxy bulges grow when galaxies merge, generally as they move along the filament. So, mergers also tend to 'flip' the alignment between the galaxy spin and the filament from parallel to perpendicular."

 

[sophiecentaur]

This is new and describes other processes that contribute to the spin of a galaxy.

That sums up what we would be after. The 'why' question is never very fruitful. All we can say we know is that there is unlikely, ever, for the component parts of a nebula to have net zero angular momentum. So you have your input spin from the very beginning and it is all about the statistics of many body interactions.