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I What keeps dark matter in galactic halos?

  1. Jun 6, 2016 #1
    I read that the cold dark matter model best fits the cosmic background radiation spectrum. I am puzzled by why most of the mapping of the distribution of dark matter shows it to be in halos about galaxies, rather than evenly dispersed with ordinary matter. The question arises from this distribution is; What pressure keeps dark matter in galactic halos and essentially out of planetary systems (Where Keplarian orbits reside, rather than the flat rotation curves of spiral galaxies)?
     
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  3. Jun 6, 2016 #2

    Orodruin

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    The question you should ask yourself is why ordinary matter clumps, not why dark matter does not. In order to become gravitationally bound to each other, the matter constituents have to lose and radiate away energy in collisions. Dark matter does not interact as strongly as ordinary matter.
     
  4. Jun 6, 2016 #3

    phinds

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    I'll expand just a bit on Orodruin's response because I think this is very non-intuitive and it gave me some trouble when I first encountered it. Gravity pulls matter (dark or otherwise) that is not in orbit around the center of a galaxy towards the center of the galaxy. As it moves from the halo towards to center it speeds up.

    Now regular matter, as Orodruin pointed out to you, clumps and dark matter does not. How do you suppose that effects the subsequent action of a particle of dark matter vs a particle of regular matter as it heads for the center?
     
  5. Jun 6, 2016 #4

    fresh_42

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    But wouldn't this only reduce the question why regular matter clumps and dark matter does not although both are gravitationally bounded?
     
  6. Jun 6, 2016 #5

    phinds

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    Your question as worded makes no sense to me. Can you rephrase? I don't get what you are asking.
     
  7. Jun 6, 2016 #6

    fresh_42

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    Why do they behave differently? If there is so much of dark matter that it even keeps galaxies in their form, why doesn't it clump due to gravity?
     
  8. Jun 6, 2016 #7

    phinds

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    Because it is in its nature not to. We don't know what it is but we know from observation how it behaves and that is how it behaves. Once we know what it is, we'll be able to figure out WHY it acts that way.

    Also, clumping has nothing to do with gravity, it has to do with electromagnetic interactions and dark matter doesn't interact via EM.
     
  9. Jun 7, 2016 #8

    Chronos

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    Dark matter does not clump, it either weakly interacts or is non-interactive via the EM force
     
  10. Jun 7, 2016 #9

    Janus

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    Nothing keeps dark matter out of planetary systems or the galaxy. Something tell me that you are being confused by the word "halo". Contrary to the common visualization as a torus-like shape, a "halo" in astronomical terms means something else. The dark matter halo for the milky way is a large sphere(Much larger than the visible galaxy) in which the visible galaxy is embedded. As a result, there is dark matter interspersed in the galaxy proper and even in the Solar system.

    The amount of DM spread out throughout the solar system works out to be equal to the mass of small asteroid. This is nowhere near enough to keep the planets from following Kepler's laws.

    SO the next question becomes: If DM is so thinly spread out, how can it have an effect on the Galaxy as a whole? The first thing you have to realise is that it is not so much a matter of DM being so thinly spread out, as a matter of the mass of the solar system being extremely condensed. To us the solar system seems to be sparsely populated and mostly empty space, but compared to the area of the galaxy surrounding it, it is very dense. If you were to take all the mass in the Solar system out to the orbit of Neptune and spread it out evenly, and then do the same for all the matter in the region out to 20 light years from Earth density of the solar system comes out to be several billion times more dense than that of the 20 light year radius region around it.

    If you then compare the density of this region of space to the density of DM in it, you also find that the former is much more dense than the later. But while the density of DM is very small, it takes up a much larger volume, which is why it can have an effect on the galaxy wide scale.

    Consider the Sun in its orbit around the center of the galaxy. If you were to compute it orbit based on the visible mass of the galaxy you would factor in the mass of the galactic bulge and the mass of the disk closer to the center than the Sun is. The distribution of the stars in the disk will cause some deviation from purely Keplerian motion, but not much. How much does DM effect this. If we were to assume that the density of DM is uniformly that of the density in the Solar system, and taking into account that DM extends both above and below the visible disk, the amount of DM works out to be a significant fraction of the total estimated mass of the whole visible galaxy. The spherical volume of DM closer to the center of the galaxy is so much larger than the volume taken up by visible matter in that same volume, that even at a density that would only add up to the equivalent of a small asteroid in the volume of the solar system, it adds up to enough mass to significantly effect the orbit of the Sun in the galaxy.

    As to why dark matter does not clump up to the degree that visible matter does. It is not gravity alone that causes visible matter to clump. It is also the fact that it interacts electromagnetically. It two visible matter particles collide, a portion of their kinetic energy is radiated away as electromagnetic energy. Thus, after the collision the result will be that they will be moving slower than they were before the collision. This makes it more likely for gravity to pull them together again or into collision with something else, causing more energy to be lost to radiation. This leads to visible matter to form structures like planets, stars and galaxies.

    DM, which does not interact electromagnetically does not have this mechanism for shedding energy. They can only interact gravitationally. This even means that MD particles don't "collide" in the way that visible matter does as even the collision between visible matter particles involve interaction between their electromagneitc fields. So DM particles approach each other, interact gravitationally, and separate moving at the same speed as before. (there wil be some small loss of energy due to gravitational radiation emission, but it would be many many magnitudes less than the equivalent loss in visible matter interaction.)

    This lack of electromagnetic interaction doesn't prevent dark matter from "clumping" to some extent, as it does tends to form halos around galaxies, it just doesn't clump as fast or as much as visible matter does.
     
  11. Jun 16, 2016 #10
    I am familiar with what you say. That still doesn't seem to explain why DM seems to aggregate only in galactic halos Are you saying, as you seem to suggest, that the CDM model is incorrect and that dark matter is so "hot" that it can only be brought into orbit by galactic scale masses and the average velocity of HDM is such that its orbital velocity keeps it in the halos?
     
  12. Jun 16, 2016 #11

    This explanation helps immensely, thank you.
     
  13. Jun 16, 2016 #12
    This explanation helps immensely. Thank you.
     
  14. Jun 17, 2016 #13
    The detection of a second black hole collision by LIGO increases the possibility that dark matter consists of stellar mass black holes (order of 10 to 100 solar masses). If that is true, then there is an interesting effect which would keep these black holes from clumping--for a while. Imagine that the state of the universe less than a million years from the Big Bang is lumpy gas with lots of scattered BHs. On a gross scale, gravity will pull everything into the expected filamentary shape. Within those forming filaments, BHs will have accretion disks (completely eaten up for small BHs within a few hundred milion years).

    These accretion disks will radiate photons which will act to push close BHs apart. If the average distance between black holes is λ, then for distances ≤ λ gravity will eventually dominate. In the early universe though, the radiation from the accretion disks, and to some extent matter ejected near the poles of BHs will push black holes apart.

    Now, lets add in SMBHs, actually we can now assume that SMBH are formed in the early universe when, at the end of inflation, there are groups of BHs that are strongly bound by gravity. These SMBHs fairly early on will be strongly radiating from accretion disks and also be ejecting mass. So dark matter BHs will get pushed away from the SMBHs by light and matter, and be attracted by gravity. But we get an interesting pattern that should be visible today. (Well the stellar mass BHs without accretion disks will be dark matter, and so invisible except to gravitational detectors.) First, right after inflation BHs clump forming SMBHs. (Accretion may be happening, but accretion disks will take time to form.) Then accretion disks push non-gravitationally bound BHs apart. Finally after the accretion are absorbed into the BHs, BH can clump again. The loss of accretion disks (and nearby matter) for non-SMBHs will make the time scale for BHs to merge on the order of billions of years, since there is no easy way to radiate (rotational) energy.
     
  15. Jun 30, 2016 #14
    Hello again. Sorry I was a bit to dense to recognize the import of your comment. I think I understand you know, at least a little better than I did. Your answer, considering DM to be particles beyond the standard model zoo, and the flat rotation curves of spiral galaxies seems to imply that the DM also rotates with the galaxies.
    Thank you for taking the time to answer my original inquiry.

    Dennis
     
  16. Jul 1, 2016 #15

    Orodruin

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    It most certainly does not. The exact velocity distribution of dark matter is unknown, but the most common assumption is a distribution isotropic in the galaxy rest frame.
     
  17. Jul 1, 2016 #16
    You are thinking about the dark matter halos and galaxies backwards. Galaxies don't draw in dark matter halos, dark matter halos draw in galaxies. The dark matter itself is what dominates the gravitational behavior of the galaxy, not the other way around.
     
  18. Jul 1, 2016 #17
    Now I am really confused. I had been thinking that each DM "particle" (?) was moving. I also was under the impression that DM interacts via gravity and is not concentrated at the nearest gravitational well but apparently moves freely throughout the DM cloud , which is itself held together by gravity (?). Since DM does not radiate in any way we can detect, it suffers no or little momentum loss and keeps on moving around in the field dominated by the "cloud" and the galaxy.. If it does gravitate it would seem that the rotating galaxy would "drag" the DM with it and over time induce a common rotation. I love a mystery.
     
  19. Jul 1, 2016 #18

    Orodruin

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    It is unclear why you think this would happen. There is no drag force. Furthermore, the dynamics of the visible matter is influenced by the dark matter, the smaller amount of matter does not alter the dark matter dynamics as much.
     
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