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Is Dark Matter Flattened by Gravity and Rotation?

-- | November 6, 2016

Question: When gravity and rotating motion get together they usually result in a flattened disc. For example, the spiral galaxies, the solar system, the rings of Saturn, and the different diameters of the Earth, around the equator and around the poles. Yet all pictures of the dark matter halo around spiral galaxies show a fairly spherical smudge around the galactic center. If dark matter is moving/rotating and subject to gravity, why does it not flatten like baryonic matter? This only depends on gravity and motion, not interaction with electromagnetic radiation. — Tom

Answer: Since the actual distribution of dark matter, to my knowledge, has not been imaged before I believe that the haloes of dark matter represented around galaxies are all artists conceptions. Note, though, that these artists conceptions are based on theoretical models for what the dark matter halo around a galaxy might look like. One of the main pieces of evidence for the existence of dark matter in galaxies is the fact that many galaxies have a flat rotation curve as a function of distance from the center of the galaxy. If the galaxy’s mass is completely determined by its visible stars, gas, and dust its rotation curve should fall as a function of distance from its center. A flat rotation curve is consistent with a roughly spherical distribution of dark matter with a flat core in a galaxy. Since we really do not have any measurements of the motion of dark matter in a galaxy, all we can discern at this point is its distribution as inferred from galaxy rotation curve measurements.

Jeff Mangum

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