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Thread: Discussion: It's a Galaxy Eat Galaxy Universe

  1. #1
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    SUMMARY: Japanese researchers using the Subaru Telescope have found a large galaxy caught in the act of consuming a smaller companion galaxy. It's a messy eater; there's a wispy trail of stars over 500,000 light-years long, which is the longest astronomers have ever seen. Examples of this kind of galactic destruction are hard to find because the consumed are usually dim dwarf galaxies. We have only indirect evidence of digested galaxies in our own Milky Way, like groups of stars traveling in an unusual trajectory.

    What do you think about this story? Post your comments below.

  2. #2
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    This is an interesting image.
    I think that the rarity of seeing this kind of galactic canabilism is an indication that large galaxies do eat up dwarf galaxies to grow, but they were probably close to their current size very shortly after they were created.
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  3. #3
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    The image is certainly very interesting, but the explanation of what we see is a bit, well, "strained" I guess.

    From the article:
    The thin band of stars extending from the dwarf galaxy both toward and away from the large elliptical galaxy reveals that the gravity of the elliptical is tidally tearing the dwarf apart. Stars that are closest to the elliptical galaxy experience a stronger pull than stars in the center of the dwarf galaxy, and stars on the opposite side experience a weaker pull. As a result, the dwarf galaxy becomes stretched and looks as if it's being pulled from two opposite directions even though there is only one galaxy doing the pulling. This effect is comparable to how two areas on the opposite sides of Earth experience high tide at the same time even though there is only one Moon tugging on Earth's oceans.
    Tides that give many thousands of lightyears long streams of stars? Doesn't it actually look like (oh, hi Duane, here's a real one, we only have this image at the moment ) the dwarf is ejected from the galaxy? Maybe the dwarf is a background galaxy, unconnected to the larger galaxy, or maybe the stream isn't connected to both?

    Cheers.

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    (oh, hi Duane, here's a real one, we only have this image at the moment )
    Ha!

    Doesn't it actually look like the dwarf is ejected from the galaxy?
    Sure, could be. Still, seems more probable that is "just passin through".

    Maybe the dwarf is a background galaxy, unconnected to the larger galaxy, or maybe the stream isn't connected to both?
    Of course, the rub here is that the red-shift of the three objects (galaxy, dwarf & stream) is the same.

  5. #5
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    Of course, the rub here is that the red-shift of the three objects (galaxy, dwarf & stream) is the same.
    Well, in Arp's world that's no guarantee , but even if they are all connected and at the same distance, and interacting (and I think they do), it still looks more like an ejection than as a collision event.

    Cheers.

  6. #6
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    The evidence for cannabalism is pretty clear. It is true that fewer of these kinds of images exist than one would expect to exlpain galaxy formation primarily by this method. There is another dwarf galaxy more recently spotted on the far side of the galactic bulge that appears to be another recent or impending meal for the milky way. The fact that SO few are seen probably is a function of how little time the dwarf is bright enough to be seen before it vanshes from view. Most likely we are able to see this one because the dwarf galaxy is relatively big compared to the sagittarius dwarf. If looking from Andromeda it is doubtful we would be able to see the sagittarius dwarf remmenants at all.
    The way I think about the total number of stars contributed to the milky way is to estimate the number in the galactic halo and double or triple that to estimate what may be orbiting right now from such cannabilism. That leaves the lion's share to much earlier events in the universe.

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    Well, I was hoping someone would explain how this "tidal" explanation works, I just don't understand how they can compare the dwarf galaxy and the larger galaxy to the Earth-Moon system. My first reaction is that these guys never fail to come up with outlandish explanations which mostly go unquestioned. For one, the galaxies aren't in orbit, which I believe is necessary for the ocean tides here on Earth. And for another, the stream is 500,000 lightyears long, how much gravity can be felt from that distance?

    Cheers.

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    Galaxy interactions are a dynamic process. It can take numerous passes in like something akin to a waltz. I believe that the gravitational pull on the smaller galaxy is not equal throughout the whole of the dwarf. The pull of the smaller galaxy on the larger galaxy is neglieable and only effects the nearest stars to the dwarf. As a result of the differing strength of the gravataional pull on the dwarf (strongest in the part closest to the large galaxy and weakest in the part furthest away,) the dwarf gets teased apart, stretched like taffy, and then wound around as it orbits the larger galaxy. The outermost stars in the larger galaxy closest to the dwarf can get pulled away also.

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    Thanks for the reply Greg,

    Galaxy interactions are a dynamic process. It can take numerous passes in like something akin to a waltz.
    I agree with your first remark, but wouldn't it take many, many billions of years for galaxies to show these streams of hundred-thousands of lightyears long? And why the thin stream? I just don't understand how this structure could form tidally.

    Cheers.

  10. #10
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    Well, I'll use a more concrete example to help illustrate what I am thinking. If you were to make a whirlpool in your sink or bathtub and then place a drop of ink in it, the effect would stretch it out in a similar manner. The closer the drop gets the more pronounced the effect is.

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    Well, I'll use a more concrete example to help illustrate what I am thinking. If you were to make a whirlpool in your sink or bathtub and then place a drop of ink in it, the effect would stretch it out in a similar manner. The closer the drop gets the more pronounced the effect is.
    Hi Greg,
    I can see what you mean, but the difference is that these galaxies are far apart, gravity is a very weak force, and they aren't rotating very fast: it would take an extremely long time for things like that.
    I just can't see gravity do it over 100,000+ lightyears. I'd like to see the effect calculated just for fun, I don't give it a big chance.

    Cheers.

  12. #12
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    Originally posted by VanderL@Nov 23 2004, 10:53 PM
    I just can't see gravity do it over 100,000+ lightyears.
    Gravity doesn't have to have acted over 100,000+ light years. The two galaxies need to have had a close encounter sufficient for the smaller one to have its outer parts tidally disrupted so some stars exceed escape velocity. This could have happened up to two billion years ago. The stuff in the forward and backward stream are stars that are no longer bound to either galaxy, and the streamers will continue to spread as time goes along, and the two galaxies pull away from each other.
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  13. #13
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    will continue to spread as time goes along, and the two galaxies pull away from each other
    And then, because of gravity, begin to fall back towards each other, in a dance covering a billion or more years that will end with the two galaxies merged.

    VanderL, gravity acts over an infinate distance with the force falling as per the square of the distance. The larger the mass, the more force acts apon a distant object, and the closer the objects, the more force there is.

    You can work out the equation yourself. The formula is:

    gravity = mass1 * mass2 * k
    ------------------- distance^2

    The unit k is a constant, which for this exercise can be set as 1. Let me know how it worked out for you.

  14. #14
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    And tidal forces go as the inverse cube of distance.
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  15. #15
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    Hi Duane, Antoniseb,

    The numbers over lightyears are extremely small, even with galaxy masses, but how do those streams keep their shape. The length is 500,000 lightyears, the width maybe several?


    Cheers.

  16. #16
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    Originally posted by VanderL@Nov 25 2004, 07:44 AM
    Hi Duane, Antoniseb,

    The numbers over lightyears are extremely small, even with galaxy masses, but how do those streams keep their shape. The length is 500,000 lightyears, the width maybe several?


    Cheers.
    What's going to change the shape?
    These are whole stars being slung around, and there is clearly no large gravitational body to make late alterations in the direction of the stream. No other force is likely to work at those distances.
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  17. #17
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    And what difference does the length of the streamers make anyway, VanderL? Remember Newton's Laws of Motion? Once an object is set in motion, it will continue in a striaght line until acted upon by another force. The stars in the streamers were pulled out of the two galaxies at a time when they passed closely by each other. Seens that close pass occurred millions of years ago.

    So again, what difference does the length of the streamers make?

    By the way, did you give the equation a try? Try altering the masses and the distance. It is really amazing to see the distance over which gravity can have an effect.

    As an aside, imagine a being on a planet orbiting a star in the streamer. The night sky would be utterly filled by the two galaxies. It would be an amazing sight.

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    And what difference does the length of the streamers make anyway, VanderL?
    Uhm, it needs something to keep it together? How would gravity keep something together that is 500,000 lightyears long and only a few across? Gravity wants to make spheres, not garden hoses.


    Cheers.

  19. #19
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    Originally posted by VanderL@Nov 26 2004, 11:20 PM
    it needs something to keep it together?
    No it doesn't. It is coming apart before our eyes. The smaller galaxy got disrupted when it went by the larger one, and stars near the outside of the smaller galaxy were tidally pulled out of the gravitational grasp of its core. Now they are unbound, spreading apart.

    If the encounter was 100 million years ago, in another 100 million years, the streamers will be twice as long, twice as wide, and one quarter as bright per square parsec.

    What gives you the impression that something is holding these streams together?
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  20. #20
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    What gives you the impression that something is holding these streams together?
    Well, the fact that the dwarf galaxy where all these stars in the streamer apparently came from, is in the middle of the stream, something is both pulling and pushing, it seems. Why don't we see swirls and clumps and things going their own way, why is it forming this stream?

    Cheers.

  21. #21
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    Originally posted by VanderL@Nov 27 2004, 12:03 AM
    Why don't we see swirls and clumps and things going their own way, why is it forming this stream?
    You'd see swirls and clumps if the streams were sufficiently self-gravitating, but they aren't. The are a bunch of stars getting even more spread out because they all came off the dwarf elliptical with a range of velocities, and generally they all came off in the two most tidally affected directions, straight ahead, and straight back, and of course the ones that didn't pull away are right in the middle.

    How much gravitational force do you think Alpha Centauri exerts on the motion of the Sun? It is non-zero, but it is also not significant. The same is true for the stars in this stream to each other.
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  22. #22
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    Hi Antoniseb,

    Thanks for trying to explain this strange interaction of galaxies.

    How much gravitational force do you think Alpha Centauri exerts on the motion of the Sun? It is non-zero, but it is also not significant. The same is true for the stars in this stream to each other.
    That part I understand, there is gravity working, but very weakly.

    The are a bunch of stars getting even more spread out because they all came off the dwarf elliptical with a range of velocities, and generally they all came off in the two most tidally affected directions, straight ahead, and straight back, and of course the ones that didn't pull away are right in the middle.
    This is the part I can't understand, do we suppose that the dwarf is orbiting the large galaxy, or is this dwarf stationary? If the stars belonged to the dwarf they had their own velocities, and after the interaction (how, when and how long, we don't know), some move towards the larger galaxy, and some others move away from both the larger galaxy and the dwarf, and not only that, it is done in a stream over enormous distances without any "breaks" or "disturbances".
    If the stars with their range of velocities is disturbed into the "tidal directions", why do all these stars behave identical? I thought that no galaxy has all it's stars moving in one direction, they orbit around the centre of gravity, so at the moment of distubance I expect some just moving away perpendicular to the core, not only in the "tidal" directions.

    You'd see swirls and clumps if the streams were sufficiently self-gravitating, but they aren't.
    We do see one big clump, the dwarf itself, but I don't see how such a structure could have formed. Maybe I'm dense, but the tidal explanation makes no immediate sense to me.

    Cheers.

  23. #23
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    Originally posted by VanderL@Nov 27 2004, 12:40 AM
    do we suppose that the dwarf is orbiting the large galaxy, or is this dwarf stationary?
    The idea is that the dwarf made a close fly-by to the big galaxy, but if it is orbiting it, it is a very long elliptical orbit. At least as likely is tha the dwarf was never gravitationally bound to the larger galaxy, and made a one-time fly-by.

    Concerning the two streams, if you think of a three body system where two of the bodies [Small & Medium] are loosely bound gravitationally, and they pass a much heavier third body [Large], let's imagine that small is trailing medium. Medium will get pulled ahead faster than small and the two will become gravitationally unbound, but both will still have too much energy to get caught by Large. The same is true if small leads medium through this situation. Now the stars that were closer to large get some extra boost and go sailing forward of medium, and the stars that were more distant to large get less of a boost, and trail behind. All keep going in more or less their original direction. That is what we see.
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  24. #24
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    Tides certainly can provide the mechanism for these streamers.

    VanderL, you said something earlier that is incorrect - galaxies DO orbit one another (the Magelanic clouds orbit the Milky Way).

    If the two galaxies get close enough together at some point in their orbit, the part of the smaller one that is nearer the larger one will move more quickly along the orbit, and lead the rest of the galaxy, while the part that is farther out will lag. Over a few orbits, the galaxy will get stretched along the direction of the orbit. This is more likely if the orbit is not circular - then the maximum stretching happens when the galaxies are closest together. It can happen on one pass, if they come close enough together.

    Something similar happens to comets. The Leonid meteor shower has outbursts that we can predict very accurately. They are made up of particles that boiled off the comet on a previous pass by the sun. They spread out along the orbit of the comet, but not perpendicular to the orbit, so they form very narrow, very long streamers. When the earth passes through one of these streamers, we get a meteor storm. Like I said, we can predict these storms very accurately, so we know these streamers are there, and they are hundreds of millions of kilometers long, but they are so narrow that one can hit the moon, and miss us.

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