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I Dumb Question: How do we know galaxies are in SSCM?

  1. Jan 4, 2017 #1
    The observed galaxy rotation curve does not match the theoretical curve derived from the model that galaxies are in steady state circular motion (SSCM). The presence of dark matter would bring the SSCM curve up to the observed curve. But what observation demonstrates that galaxies are in SSCM in the first place?
  2. jcsd
  3. Jan 4, 2017 #2
    Are you asking how do we know how fast galaxies are rotating? We measure the red-shift of the stars on either edge of the galaxy. Stars moving towards us shift blue and those moving away shift red. We know that the stars are putting out a lot of energy at the hydrogen spectrum, so we can measure how far the frequency got shifted and calculate it's relative speed with very high accuracy.
  4. Jan 4, 2017 #3
    No, my question is not about how fast galaxies are rotating. It is: How do we know galaxies are in SSCM, which is the model used to generate theoretical rotation curve. Make sense?
  5. Jan 4, 2017 #4
    Can you provide a link to what you are describing? Orbital dynamics is described by the mathematics of General Relativity and they certainly aren't circular, they're elliptical. We know galaxies gravitate this way because the GR equations don't care what's actually there: matter, energy, dark matter... it's all the same as far as gravity is concerned and gravity is the only force that applies to galaxies.

    The orbits of stars in the galaxy is exactly the same equation as the orbit of planets around the sun. The only difference is that there isn't one point source of gravity in a galaxy like there is for a star (the sun can be thought of as a point when talking about distances at this scale,) but the way it bends space and how that affects the planets is the exact same process that works on galaxies.
  6. Jan 4, 2017 #5
    The elliptical paths is not the issue. Here is a link. It says this: "We know that the Sun is moving in a circle around the center of the Milky Way. What keeps the Sun moving in this circle instead of flying away? Gravity, of course."

    My question is: How do we "know that the Sun is moving in a circle" (or ellipse)?
  7. Jan 4, 2017 #6
    It may be the same equation but that doesn't demonstrate that stars are in elliptical orbits, right?
  8. Jan 4, 2017 #7


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    In order for a galaxy to not be in SSCM, it would have to be unstable: it'd have to be in the process of coalescing or dispersing. But this doesn't make any sense for spiral galaxies with well-defined structure, which is where this model is used. Irregular galaxies can't really be described as obeying SSCM, but all galaxies that settle into a spiral pattern should.

    You can do something similar with elliptical galaxies, but the motions of the stars aren't in a single plane as they (mostly) are with spiral galaxies.
  9. Jan 4, 2017 #8
    Can you explain why a "well-defined" structure means the galaxy cannot be dispersing? It isn't real clear to me why a spiral galaxy must be in SSCM, by virtue of its structure? Not saying this is false, just don't understand why this is true.
  10. Jan 4, 2017 #9


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    It ultimately comes down to the fact that energy is (mostly) conserved in a system like a galaxy. Energy isn't conserved in general in General Relativity, but galaxies mostly obey Newtonian gravity, which obeys strict energy conservation.

    So what happens is that as a galaxy forms, it gradually settles into a shape. I believe that the prevailing theory is that the amount of dust in the galaxy determines what shape that will be: galaxies with a lot of dust experience significant amounts of friction, which brings the orbits of the stars into a single plane, creating a spiral galaxy. If there isn't much of any dust, then there isn't any significant friction and the stars' orbits randomize as they exchange energy between one another. This creates an elliptical galaxy, where the stars all have nearly-elliptical orbits, but in wildly different directions.

    The galaxies that are relevant here are the spiral galaxies. In order to settle into the spiral pattern, they had to experience a large number of orbits around the center of the galaxy. This means that the stars in the galaxy are in a roughly stable pattern, leading to nearly-circular orbits all in the same plane.

    And by the way, this is measurable for nearby spiral galaxies, by measuring the redshifts of different parts of the same galaxy. So it's not just conjecture based upon theory.
  11. Jan 4, 2017 #10
    But the measured redshifts do *not* demonstrate a SSCM. What it looks like you are saying is that the galaxy must be SSCM by virtue of its structure *and* how we think it originated.
  12. Jan 4, 2017 #11
    Oh, well, we can observe it or calculate it. We've watched the sky long enough to know our relative position and vector within the Milky Way. We know how much mass the galaxy has too, so we can calculate the geometry of spacetime. From there we can calculate backwards and watch the sun go backwards around the galaxy.

    We mostly look at far away galaxies though, we know those are going in circles because stars on one side move towards us and the stars on the other side are going the other way. We know that from redshift. The only explaination for that is an orbit, or else they would fly apart.
  13. Jan 4, 2017 #12
    AGain, dumb question, but why does "one side move towards us and the stars on the other side are going the other way" mean they are "going in circles"? You mentioned that we know that from redshift, but doesn't the redshift merely give us the current velocities? You say this means they must be in an orbit, "or else they would fly apart." Why is that not possible?
  14. Jan 4, 2017 #13


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    How about this: we can observe a vast number of galaxies, and in general, they are not in a process of disintegration. Whereas the discrepancy in measured velocities and escape velocities calculated from baryonic matter content would allow the galaxies to disperse very fast.

    So, either we assume these are stable structures, or the universe conspired to show us galaxies across vast distances, and (taking into account light travel time) in various stages of their 'lives', that just happen to look like stable structures to us today.
  15. Jan 4, 2017 #14
    Yes, that seems like a good argument. My only question is: just how consistent are those structures? When I look at those photos, there seems to a lot of variation with those galaxy shapes, sizes, etc. So just how obvious would that conspiracy have to be? But I agree, given the kind of consistency that you are talking about, that seems like a pretty good argument.
  16. Jan 4, 2017 #15


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    Most spiral galaxies have very similar structures, with the primary differences being down to the size of their central bulges, or whether they are barred spirals. Galaxies that have newly-formed or recently undergone mergers tend to have much more irregular shapes, but this doesn't apply to most galaxies.
  17. Jan 4, 2017 #16
    Because we can tell how old these galaxies are. If they were flying apart, we'd see galaxies dispersing and they don't. As far back as we can see, galaxies act like we expect them too.

    We can also see the redshift of all of the other stars too. You see a smooth gradient in the redshift from one side to the other. That's only possible if the vectors change slightly the same way as you go from one side to the other. That shows the relative motion of everything in the galaxy.

    The look like this.

    Attached Files:

  18. Jan 4, 2017 #17
    Nice graphic.
  19. Jan 7, 2017 #18


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    We observe numerous spiral galaxies out to billions of light years. That fact alone appears to defang your apparent argument. We fit models to observation not observation to models.
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