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#1 skyguy88

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Posted 16 August 2013 - 12:16 AM

I was asked why spiral galaxies have the spiral form at a recent outreach event. Lead me to realize how much I don't understand. A few examples:

Did galaxies begin as giant rotating H, He,(+dark matter) clouds? If so, where did the angular momentum come from? Can a galaxy form without rotation?

Do most stars rotate about an axis parallel to the galaxy axis of rotation?..and in the same sense?

Can stars form from a non-rotating cloud?

How do globular clusters relate to the formation of the galaxy?

Were central black holes instrumental in the formation of galaxies?...in what role?

How does the spiral form originate?

Answers to any of these Q's would be most welcome.

Thanks,

Bill

#2 StarWars

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Posted 16 August 2013 - 01:38 AM





Maybe a galaxy begins as a massive blue star. The blue star is rotating when it super nova and blows off star and planet making material. A black hole is created at the center of the galaxy. With time the spiral arms create suns, planets and solar systems.

#3 GlennLeDrew

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Posted 16 August 2013 - 03:06 AM

Bill,
The answers to your laundry list of questions could fill a small book. :grin:

About galaxy formation. It might be unrealistic to picture a monolithic cloud; a multitude of clouds contracting toward the mean center seems more plausible. The initial large extent, and expected difference in velocities, essentially guarantees some kind of net rotation when the system contracts to a reasonably stable configuration. Gas clouds have high 'friction', and as they collide in a system having net rotation, they fairly quickly arrange themselves into a highly flattened disk with more or less circular motion. A good example is Saturn's rings, the particles are suitably large with respect to their separation that collisions are frequent, thus averaging out their energies to the point of placing all on remarkably circular orbits and very small velocity dispersion.

(The concept of velocity dispersion is fundamental to understanding the dynamics of so much, from atoms in a gas, to stars and gas clouds in galaxies, and galaxies in clusters.)

During the early contraction process, clouds on *many* scales will be collapsing to form stars, the vast majority as members of clusters and larger associations. This is ongoing all the while as the galaxy is a-building. Some of these earlier massive clusters become globulars, perhaps most even starting out as small galaxies in their own right.

The earlier generation of stars, clusters and small galaxies which formed during the contraction phase will have a wide variety of direction, this making up the extended halo of the galaxy.

Later generations of stars and clusters which formed after a more flattened disk of gas developed will have more uniform motion, confined more to a plane and on nearly circular orbits. It's here, where the gas has largely settled, that ongoing star formation is possible. And so unlike the halo, where we find mostly old, metal-poor stars, in the disk we find stars which have formed from continually re-processed gas which has become ever more polluted with the effluvia of previous stars which have puffed, belched or blasted much of their remains back into the mix.

With the development of a highly flattened disk, and perhaps the formation of an asymmetric central bulge (a bar), a density wave pattern can develop. This leads to the organization into spiral arms, where the gravitational potential minima cause a crowding together of material and initiating a higher rate of star formation.

The central black holes found in galaxies is only a result of the congregation of matter in close confines. It's a phenomenon resulting from, not initiating, galaxy formation.


This is as brief a summary I can cook up which touches upon your questions.

#4 Ira

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Posted 18 August 2013 - 05:08 PM

Note this new paper that shows that galaxies had their mature shapes that we see today as early as 2.5 billion years after the big bang. Rather surprising to me,

http://www.scienceda..._source=feed...

/Ira

#5 GlennLeDrew

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Posted 18 August 2013 - 05:40 PM

If we take a characteristic orbit period of material about a galaxy as 250 million years, then galaxies 'settled down' to near their final configuration in some 10 orbit periods. That does seem pretty quick.

#6 Ira

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Posted 18 August 2013 - 06:06 PM

Especially since in their early stages they must have been pretty chaotic. Perhaps the universe is alot older than we believe! :)

/Ira

#7 llanitedave

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Posted 18 August 2013 - 07:08 PM

Even if they were just fragments of converging galaxies-to-be, I don't see any reason for them not to show familiar structures. Remember how nature is similar on so many different scales. If the protogalaxies were rotating and making stars and containing gas, then spiral structure would have been a natural result.

Galaxy clusters are often found stretched along lineations, as if there's a single dominant stream. I'd hypothesize that the protogalaxies that came together to form the final products were arranged in similar linear structures, which would preserve the disk structure of the final rotating galaxy. Since the cosmos back then had much less volume, and protogalaxies were much more densely spaced, it may not have had to take that long.

#8 GlennLeDrew

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Posted 18 August 2013 - 10:06 PM

Dave,
The 'preservation' of structure after mergers is hardly the norm. Look at the mess resulting after two spirals collide. Especially if their axes of symmetry are far from parallel, the result soon becomes more an elliptical, perhaps with a disk of gas settling down (like the Sombrero and the Hamburger, perhaps?). The expected variation in the sense of rotation among galaxies in a cluster suggests that mergers should more often result in a mixing of stellar orbits, and so a more elliptical configuration when tidal structures settle down.

Depending on the velocity, directions of rotation, and other impact parameters for two impacting spirals, the gas can be largely stripped from each as they pass through each other. The gas clouds will slam together and effectively come to a screeching halt, the stellar systems continuing on, albeit strongly disturbed by tidal forces. The shocks induced in the gas will initiate ferocious star birth, perhaps making a new irregular galaxy.

#9 Qwickdraw

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Posted 19 August 2013 - 03:54 PM

If we take a characteristic orbit period of material about a galaxy as 250 million years, then galaxies 'settled down' to near their final configuration in some 10 orbit periods. That does seem pretty quick.


I have always thought the given estimate of the age of the universe has not been nearly enough time for galaxy formation to the extent we see today. As you suggest, 10 revolutions? And that is at the current state of order. How long did a revolution take in early galaxy formation? It is my understanding that as they spin faster as they are gravitationally condensed.

#10 GlennLeDrew

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Posted 19 August 2013 - 04:07 PM

10 revolutions is over 2.5 billion years. After ~13 billion years, galaxies will have rotated over 50 times.

The orbital velocity of material around spiral galaxies does not vary a great deal from small to large. Moreover, after a fairly linear increase in orbital velocity in the central bulge, throughout the rest of the disk the velocity is almost constant. Unlike a Keplerian potential (like the solar system), where the bulk of the mass is in a central body and orbital velocity falls off exponentially with distance from it, a galaxy's vast halo alters the potential so that orbital velocity is as fast in the outer disk as in the middle and inner.

#11 llanitedave

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Posted 20 August 2013 - 10:28 PM

Dave,
The 'preservation' of structure after mergers is hardly the norm. Look at the mess resulting after two spirals collide. Especially if their axes of symmetry are far from parallel, the result soon becomes more an elliptical, perhaps with a disk of gas settling down (like the Sombrero and the Hamburger, perhaps?). The expected variation in the sense of rotation among galaxies in a cluster suggests that mergers should more often result in a mixing of stellar orbits, and so a more elliptical configuration when tidal structures settle down.

Depending on the velocity, directions of rotation, and other impact parameters for two impacting spirals, the gas can be largely stripped from each as they pass through each other. The gas clouds will slam together and effectively come to a screeching halt, the stellar systems continuing on, albeit strongly disturbed by tidal forces. The shocks induced in the gas will initiate ferocious star birth, perhaps making a new irregular galaxy.


It looks to me like it works both ways, Drew. I know there are lots of giant ellipticals near the centers of clusters -- certainly the result of multiple mergers. But it seems like big spirals have often built up through mergers as well. And I'm really not aware of any giant irregulars.

The main correlations I see is that giant ellipticals tend to be concentrated deep within large clusters, where the impacts from their constituents would have potentially come from all directions, while spirals tend to be more generally distributed: maybe instead of not having experienced any collisions, their collisions have been from more restricted directions? Seeing spirals lined up along narrow paths makes me suspicious about that.

#12 GlennLeDrew

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Posted 20 August 2013 - 11:47 PM

Any spiral which has a regular structure will not have suffered a significant impact/merger in the recent past. There could have been the cannibalizing of smaller galaxies, such as dwarf ellipticals, which do not much affect the larger spiral. When two galaxies of anything like similar mass meet, no matter the impact parameters there will result notable tidal disruption at least, taking no small amount of time to settle into the new, stable configuration.

#13 Ira

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Posted 21 August 2013 - 05:34 AM

Does the Hubble Ultra Deep Field show us galaxies at different eras of formation or does it not look far enough back in time? I seem to remember some very odd-looking, non-modern galaxies in the UDF.

/Ira

#14 StupendousMan

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Posted 23 August 2013 - 01:01 PM

You remember correctly. You might read these papers (among many others):

http://adsabs.harvar...ApJ...722.1895E
http://adsabs.harvar...ApJ...631...85E

#15 Ira

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Posted 24 August 2013 - 06:30 PM

That's cool. Thanks! (Especially the second).

/Ira

#16 vickster339

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Posted 27 August 2013 - 02:54 AM

You are asking good questions, might I encourage you to take it to another level. How do galaxies form at all from the homogeneous background of the big bang while maintaining the metallicity discrepancies we observe?

How can massive stars establish a hydrostatic equilibrium of plasma before ejecting all of their fuel during a localized ignition?

Those two should be enough for now.

#17 llanitedave

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Posted 27 August 2013 - 09:50 PM

I don't think anyone has said that the Big Bang's background was homogeneous.

#18 GregLee1

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Posted 27 August 2013 - 09:59 PM

I don't think anyone has said that the Big Bang's background was homogeneous.

You mean the early universe?

In its early days, the universe was extremely smooth and homogenous... but not quite perfectly so.
The Cosmic Microwave Background



#19 llanitedave

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Posted 27 August 2013 - 10:22 PM

I guess homogeneity is in the eye of the beholder.

#20 Jarad

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Posted 28 August 2013 - 05:59 AM

Homogenous is a relative term...

Jarad






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