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Thread: Spiral galaxies have very flat disks. Why?

  1. #1 Spiral galaxies have very flat disks. Why? 
    Forum Bachelors Degree PetTastic's Avatar
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    We see lots of images of galaxies tidaly interacting and coliding.
    Stars travel past each other and must nudge each others orbit around the galaxy.
    Radiation pressure from powerful stars and supernova should blow dust away from the disk.

    If the dark matter halo is spherical, and the gravitational pull of the disk look comparatively small, what keeps the disk flat and thin?

    Does MOND have any explanation?


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    AI's Have More Fun Bad Robot's Avatar
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    Quote Originally Posted by PetTastic View Post
    We see lots of images of galaxies tidaly interacting and coliding.
    Stars travel past each other and must nudge each others orbit around the galaxy.
    Radiation pressure from powerful stars and supernova should blow dust away from the disk.

    If the dark matter halo is spherical, and the gravitational pull of the disk look comparatively small, what keeps the disk flat and thin?

    Does MOND have any explanation?
    I found a link that has several answers. However, I'm not sure I can really get behind any of them.

    astrophysics - Why are some galaxies flat? - Physics Stack Exchange

    In the early universe most of the stars were in smaller clusters. Somehow they start orbiting each other or a common center of gravity and when that happens most of the stars are moving or orbiting in the same direction. I have my own ideas about the supermassive black holes that seem to be a key feature of all mature current day galaxies, which I won't get into for this topic. However, as galaxies merge and become larger, the rotation of the stars in the larger galaxy will create the most gravitational drag where the stars are denser (closer together). This will be the galactic plane, and over time the gravity of the main disk will pull any out of alinement stars in to the main disk.

    Then why are some galaxies elliptical? It's been observed that most of the elliptical galaxies are very large much larger than the Milkyway, the point being that when two or more very large galaxies merge, you have a great deal of star disruption which destroys the disk of the merging galaxies, creating the elliptical shape of the new galaxy with maybe a trillion or more stars orbiting in every which direction which makes it unlikely they will ever pull together into a flat spiral disk. As the universe gets older and more large galaxies merge together, we will have an increasing number of very large elliptical galaxies and a corresponding decrease in the number of spiral disk galaxies.


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  4. #3  
    Time Lord
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    Still leaves the spherical dark matter halo unexplained, but at least it explains the rest.
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  5. #4  
    Forum Radioactive Isotope sculptor's Avatar
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    some of this has already been gone over here:
    galaxies, disks & spirals
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    Malignant Pimple shlunka's Avatar
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    Because if they weren't flat they'd be called spherical galaxies.
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    AI's Have More Fun Bad Robot's Avatar
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    Quote Originally Posted by kojax View Post
    Still leaves the spherical dark matter halo unexplained, but at least it explains the rest.
    In our own solar system isn't the Oort cloud some what spherical around our solar system. I don't see why dark matter can't follow the same format for matter outside the galactic body of stars.
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    To understand galaxy formation, and the evolution from clusters, to ellipticals, to bar, to spiral galaxies read this:
    magneticreconnectiongalaxy.com
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    Genius Duck Moderator Dywyddyr's Avatar
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    Quote Originally Posted by Eleven11 View Post
    To understand galaxy formation, and the evolution from clusters, to ellipticals, to bar, to spiral galaxies read this:
    magneticreconnectiongalaxy.com
    Alternatively, applying science might be better.
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  10. #9  
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    Quote Originally Posted by Eleven11 View Post
    To understand galaxy formation, and the evolution from clusters, to ellipticals, to bar, to spiral galaxies read this:
    magneticreconnectiongalaxy.com
    Please don't bring crank websites into a serious question about science.
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    Forum Bachelors Degree PetTastic's Avatar
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    I think the issue is why are the disks still flat.
    When they get warped or distorted by collisions or close approches withother galaxies what restores them to nice thin flat disks?


    As most galaxies exist in clusters, they must have undergone many major interactions, and then the disk repaired themselves.

    I am still reading the other threads mentioned above, but so far they don't contain anything to explain this.
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  12. #11  
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    The link Bad Robot provided answers the O.P and post number 10.
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  13. #12  
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    Quote Originally Posted by Bad Robot View Post
    Quote Originally Posted by PetTastic View Post
    We see lots of images of galaxies tidaly interacting and coliding.
    Stars travel past each other and must nudge each others orbit around the galaxy.
    Radiation pressure from powerful stars and supernova should blow dust away from the disk.

    If the dark matter halo is spherical, and the gravitational pull of the disk look comparatively small, what keeps the disk flat and thin?

    Does MOND have any explanation?
    I found a link that has several answers. However, I'm not sure I can really get behind any of them.

    astrophysics - Why are some galaxies flat? - Physics Stack Exchange

    In the early universe most of the stars were in smaller clusters. Somehow they start orbiting each other or a common center of gravity and when that happens most of the stars are moving or orbiting in the same direction. I have my own ideas about the supermassive black holes that seem to be a key feature of all mature current day galaxies, which I won't get into for this topic. However, as galaxies merge and become larger, the rotation of the stars in the larger galaxy will create the most gravitational drag where the stars are denser (closer together). This will be the galactic plane, and over time the gravity of the main disk will pull any out of alinement stars in to the main disk.

    Then why are some galaxies elliptical? It's been observed that most of the elliptical galaxies are very large much larger than the Milkyway, the point being that when two or more very large galaxies merge, you have a great deal of star disruption which destroys the disk of the merging galaxies, creating the elliptical shape of the new galaxy with maybe a trillion or more stars orbiting in every which direction which makes it unlikely they will ever pull together into a flat spiral disk. As the universe gets older and more large galaxies merge together, we will have an increasing number of very large elliptical galaxies and a corresponding decrease in the number of spiral disk galaxies.
    The gravitational drag argument does not work very well for fixing disks after collisions because once the disk is distorted out of its plane, there is no grag in the plane of the disk any more.

    There is very little evidence that eliptical galaxies are the results of collisions.
    If you consider the half way state Lenticular galaxies, they often still have a perfect disk with no indication of damage.
    Lenticular galaxy - Wikipedia, the free encyclopedia



    As the interstellar medium of most galaxies is 1010 times thinner than air ( a few atoms per cm3)it going to present very little drag on a star at 1030 even over billions of years.

    I like the interesting comparison to the rings of Venus, in one of the threads, but galaxies lack the obvious damping mechanism of physical impact with the rings.
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  14. #13  
    AI's Have More Fun Bad Robot's Avatar
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    Quote Originally Posted by PhDemon View Post
    Quote Originally Posted by PetTastic View Post
    As the interstellar medium of most galaxies is 1010 times thinner than air ( a few atoms per cm3)
    Nitpick! At atmospheric pressure there are ~2.5 x 1019 molecules cm-3 so your factor of 1010 is a little out! (Sorry to be a pedant but it's a number I use every day)...
    Just thought I'd fix your posts a bit.
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  15. #14  
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    Quote Originally Posted by Bad Robot View Post
    Quote Originally Posted by PhDemon View Post
    Quote Originally Posted by PetTastic View Post
    As the interstellar medium of most galaxies is 1010 times thinner than air ( a few atoms per cm3)
    Nitpick! At atmospheric pressure there are ~2.5 x 1019 molecules cm-3 so your factor of 1010 is a little out! (Sorry to be a pedant but it's a number I use every day)...
    Just thought I'd fix your posts a bit.
    The interstallar medium in molecular clouds can get up to 1012 in active galaxies. (according to my notes)

    Not that the numbers matter that must it still produces only a tiny drag on a star.

    How about this
    One way to increase the gravitional drag and transfer the kninetic energy into the interstellar gas damping it, is to have interstellar rocks.
    (Back to that Oort cloud stuff again. Sorry. Is the idea of the Oort cloud dated?)


    A stella mass of rocks between the stars, would be millions of times more effective at damping movement than a star.
    Massive surface area in contact with the gas, and far more mobile / near moving stars to absorb gravational energy from them.
    Not done any maths on this yet but to me looks like a winner at first glance.
    Last edited by PetTastic; August 1st, 2013 at 10:17 AM. Reason: spelling
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  16. #15  
    AI's Have More Fun Bad Robot's Avatar
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    Quote Originally Posted by PetTastic View Post
    How about this
    One way to increase the gravitational drag and transfer the kinetic energy into the interstellar gas damping it, is to have interstellar rocks.
    (Back to that Oort cloud stuff again. Sorry. Is the idea of the Oort cloud dated?)


    A stellar mass of rocks between the stars, would be millions of times more effective at damping movement than a star.
    Massive surface area in contact with the gas, and far more mobile / near moving stars to absorb gravitational energy from them.
    Not done any maths on this yet but to me looks like a winner at first glance.
    I used the Oort cloud only as an example that matter outside of the system while still gravitationally connected to the system is not necessarily going to line up in the plane or flat orbital disk like the planets do. The same might be said about the dark matter surrounding the galaxy.

    Also when I used the term gravitational drag, I wasn't talking about damping movement. I was thinking more about changing the orbits, but I'm sure that only works that way when a large spiral galaxy merges with smaller star clusters. When say two large galaxies like the Milkyway & Andromeda merge the total star count might go over a trillion and they will all be disrupted. Some of the orbits may be going in the opposite directions or over the polls in other words every which way possible. At this point it doesn't seem possible that all those stars could ever pull together into another spiral galaxy.

    Ever wonder why all elliptical galaxies are so much larger than spiral galaxies? I'm betting there is a star count limit that merging spiral galaxies can't recover from ( reforming another spiral galaxy ). Our Milkyway is only 200,000 light years in diameter and the largest elliptical is 6,000,000 light years in diameter. I'm not sure what the largest spiral galaxy is, but I'm sure it's just a baby compared to that monster elliptical.
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  17. #16  
    Forum Bachelors Degree PetTastic's Avatar
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    Quote Originally Posted by Bad Robot View Post
    Quote Originally Posted by PetTastic View Post
    How about this
    One way to increase the gravitational drag and transfer the kinetic energy into the interstellar gas damping it, is to have interstellar rocks.
    (Back to that Oort cloud stuff again. Sorry. Is the idea of the Oort cloud dated?)


    A stellar mass of rocks between the stars, would be millions of times more effective at damping movement than a star.
    Massive surface area in contact with the gas, and far more mobile / near moving stars to absorb gravitational energy from them.
    Not done any maths on this yet but to me looks like a winner at first glance.
    I used the Oort cloud only as an example that matter outside of the system while still gravitationally connected to the system is not necessarily going to line up in the plane or flat orbital disk like the planets do. The same might be said about the dark matter surrounding the galaxy.
    There looks to be two ways for the disk to form or reform, either by changing the orbit of star as they pass through the disk so that their orbits follow the plane of the disk.
    Or damp the vertical motion of stars each time they oscillates up and down through the plane of the disk.
    It is difficult to get any model to achieve the first by random interaction, so I am concentrating on damping effects.

    Damping from dark matter drag has big problems in my view.
    If the dark matter does not have the same rotation curve as the stars in the galaxy, the orbits of stars would decay and everything falls into the central black hole.
    If the dark matter does have the same rotation curve, and there is dark matter drag, then Andromida would turn the Triangelum galaxy into a smear on the intergalatic landscape.

    Quote Originally Posted by Bad Robot View Post
    Also when I used the term gravitational drag, I wasn't talking about damping movement. I was thinking more about changing the orbits, but I'm sure that only works that way when a large spiral galaxy merges with smaller star clusters. When say two large galaxies like the Milkyway & Andromeda merge the total star count might go over a trillion and they will all be disrupted. Some of the orbits may be going in the opposite directions or over the polls in other words every which way possible. At this point it doesn't seem possible that all those stars could ever pull together into another spiral galaxy.
    As far as I can see without damping, stars star clusters would just continue to oscilate up and down through the disk for hundreds of millions of years, until their obital energy is transfered to the disk. Increasing the verticle oscilations all the stars in the disk, making it thicker.

    Quote Originally Posted by Bad Robot View Post
    Ever wonder why all elliptical galaxies are so much larger than spiral galaxies? I'm betting there is a star count limit that merging spiral galaxies can't recover from ( reforming another spiral galaxy ). Our Milkyway is only 200,000 light years in diameter and the largest elliptical is 6,000,000 light years in diameter. I'm not sure what the largest spiral galaxy is, but I'm sure it's just a baby compared to that monster elliptical.
    There are some big spirals (unclear the will remain spirals) NGC 6872 on the large side, an some dark galaxies look to have a spiral structure (Malin 1), it is hard to tell how big they get.

    Still looking at this interstellar rocks idea, doubling the visible mass of the disk by adding 1kg sized rocks between the stars seems to increase damping effect in my dirty models my many orders of magnitude. ( not millions as I said in last post).
    About [9.4x10-12 kg/km3 edit] or interstellar iron/rock has a massive effect on damping and transferring energy to the interstellar medium.
    Last edited by PetTastic; August 2nd, 2013 at 06:18 AM. Reason: Had completely wrong number for density of rocks ???
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