Why are spiral galaxies generally planar?

[newtonsdad]

Hi Pf,

I've searched through the forums for an answer to this question and have found bits and pieces but not a full answer. Why are spiral galaxies planar (I know there are exceptions to the rule, I'm just trying to understand the why). From what I understand, it is a bit of a battle between gravity, centripetal acceleration and conservation of angular momentum. I understand these concepts but I can't figure out why this results in a planar galaxy.

I was hoping someone might be able to provide a fuller explanation. Some of the more specific points I don't understand include:

- if the gas is a spheroid originally, what brings the material down from the "top" of the sphere into the plane of rotation?

- why does the mass start spinning in the first place?

- do galaxies not collapse to a point or break apart because the gravitational and inertial forces are balanced? if not, what is happening to keep the general size constant?

Thank you very much, I really appreciate any help!

 

[marcus]

Here's a technical article. Not especially understandable:
http://arxiv.org/abs/astro-ph/0202466

They did computer simulations of "bottom-up" galaxy formation by the gradual accumulation of smaller clumps clusters and gas-clouds.

Galaxy formation STARTS from a universe that already has a filamentary cobwebby structure based on random pockets of overdensity and underdensity. random accumulations falling headlong towards each other.
Computer simulations of this early universe process match reality remarkably well, even after the largescale cobwebby strands have condensed into clusters of galaxies we still see the traces and remants of that early structure

Google "TED smoot" and watch the last 3 or 4 minutes of George Smoot's talk to the TED meeting in SantaMonica.

Starting from random cobweb strands falling towards each other you are bound to get SOME clumps where there is some net angular momentum. The collapse is asymmetrical.

In the very simplest imaginable picture suppose two clouds of crud collide, radiate some collision energy and remain gravitationally bound. If they don't collide exactly head-on, there will remain some spin in the result.

What http://arxiv.org/abs/astro-ph/0202466 tells you is that once you have a heap of crud that is rotating, and more and more gas pours into it getting swept around and trapped gravitationally (radiating excess energy so it can remain bound) the thing can evolve to disk.

They show pictures of the results of their computer simulation.

In fact their simulation showed that even after collision/merger with another galaxy has disrupted both disks the merged thing can still recover a disk shape. Not always, and not immediately, but in some cases it does.

The pictures are not especially attractive or inspiring, but they show a physical process of spiral galaxy formation.

Eventually after many collisions and mergers you get a "blob" that doesn't form into a disc. These giant blob galaxies are called "elliptical" galaxies.
==================

The direct answer to your question is, I think, that we do not completely understand the process of spiral galaxy formation. It is still an active research topic. Computer simulations have helped. There are still unexplained things like why there are so MANY spiral galaxies. One would have thought that by now collision/merger would have caused more of them to be ellipsoidal blobs. The models need refining so that the predict the right distribution of shapes and sizes. But it's pretty impressive, I think, how much is already understood.

 

[H2Bro]

Im a layman, but I do know that the gas is more likely highly irregular initially, as small pockets attract together to form larger clouds and so forth. Rotation would begin as clouds of stars miss each other but remain gravitationally bound which starts the spin.

As for constant size, I believe the densest areas of gas - where stars form along spiral arms - rotate about the center as in an orbit, though orbiting faster than expected towards the outside of the arms (which we attribute to dark matter. )

Interestingly there is some evidence that above and below the galactic plane very large magnetic fields channel and recycle gas thrown off in supernovae, funnelling it back into star forming areas (http://www.physorg.com/news/2011-12-all-sky-magnetic-fields-milky-highest.html). Might this help to explain the persistent shape.

for good measure here is a gratuitous galaxy formation video. http://www.youtube.com/watch?feature=endscreen&NR=1&v=n0jRObc7_xo

 

[phinds]

One of the things I've noticed is that the angle of spiral galaxies as seen from Earth is totally random. This suggests to me that the random mass accretion that happens to gain the largest accumulation at first is "built on" by the rest of the process and the whole thing ultimately takes on the angle of that initial largest mass, unless a more massive blob joins the game, but the point is that the rotation of that initial blob (and possibly perturbed by subsequent blobs) causes it to be planar and this gradually becomes the shape of the whole thing.

The like given above by H2Bro is quite interesting, but I would find it MUCH more so if it gave a better 3D perspective instead of appearing so 2D.

 

[Chronos]

The short answer is spiral waves.

 

[H2Bro]

Is another way to describe the planar shape to say, that areas outside the plane of rotation cannot be balanced in centripetal acceleration and gravitational attraction while maintaining the same angle of elliptic, and additionally material in off-elliptic rotations gets caught up and trapped when it moves through the plane. Is this halfway accurate?

 

[Chronos]

Lin and Shu came up with the spiral density wave idea in the 60's, as I recall. It was a brilliant insight that has since been validated using numerical simulations.

 

[marcus]

The original question was not about what causes the spiral arm structure.(The prevailing explanation for that is the density wave model proposed by Lin and Shu see e.g. Wikipedia.)
http://en.wikipedia.org/wiki/Spiral_galaxy
Nice graphic results of computer simulations here:
http://en.wikipedia.org/wiki/Density_wave_theory

Instead, the original question is why the overall pancake form. Planar. That was not addressed by the spiral density wave model which already assumes stars orbiting center in a roughly common plane.

Hi Pf,

... I understand these concepts but I can't figure out why this results in a planar galaxy.

I was hoping someone might be able to provide a fuller explanation. Some of the more specific points I don't understand include:

- if the gas is a spheroid originally, what brings the material down from the "top" of the sphere into the plane of rotation?

- why does the mass start spinning in the first place?
...

The short answer is spiral waves.

I don't think so Chronos. You seem to be answering a different question that wasn't asked in this thread. We've talked about spiral structure and density waves in other threads, as I recall. The question here is about planarity.

Lin and Shu came up with the spiral density wave idea in the 60's, as I recall. It was a brilliant insight that has since been validated using numerical simulations.

Yes indeed! But this addresses the question of spiralness and does not address the question of pancakeness

 

[Chronos]

The planar part appears so simple I have probably completely missed something. I assume it is merely a consequence of centrifugal force. If you take a gas cloud bound solely by gravity and impart spin to it, it flattens out.

 

[marcus]

The mechanism of galaxy formation continues to be an interesting and unresolved problem.
Simon White co-authored this recent contribution.
http://arxiv.org/abs/1112.2220
The Origin of Disks and Spheroids in Simulated Galaxies
Laura V. Sales, Julio F. Navarro, Tom Theuns, Joop Schaye, Simon D. M. White, Carlos S. Frenk, Robert A. Crain, Claudio Dalla Vecchia
(Submitted on 9 Dec 2011)

This does not have to do with spiral arm formation. Frank Shu's density wave explanation of that from the 1970s still stands as far as I know.

The origin of planarity is still being studied and discussed. Earlier models of gas accretion from the 1990s have been overturned. One has to take into account the filamentary DM structure and the gas cloud halo dynamics surrounding the galaxy. One has to account for the statistics---we see both planar and spheroidal galaxies of various sizes. What accounts for the proportion of planar ones?
Various explanations/mechanisms have been proposed and tested by computer simulation to see if they give realistic results. And they are still being challenged and discarded and replaced by better explanations/mechanisms. It's an interesting research area.

Simon White et al planar versus spheroid paper concludes with this final sentence:

Until simulations can reproduce not only the properties of individual systems, but the full statistical distribution of galaxy morphologies and their dependence on mass and environment, it is likely that a full understanding of the origin of galaxy morphology will remain beyond reach.

 

[Chronos]

The nature of the dark matter halo probably has some influence - my .02c.