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Astronomy and Cosmology
Astronomy and Astrophysics
What keeps dark matter in galactic halos?
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[QUOTE="Janus, post: 5493136, member: 4"] 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 [i]is[/i] 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 realize 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. [/QUOTE]
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What keeps dark matter in galactic halos?
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