DM Halo Movement: Predicted vs Observed

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In summary, dark matter is thought to impact the shape of galaxies, but not necessarily in a way that would cause them to look different from galaxies without dark matter.
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jhe1984
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Okay - and this is most likely a gross oversimplification - but (for galaxies thought to have DM halos), the predicted motion/predicted spin of the galaxy would be much tighter, while the observed motion (with the DM accounted for) is actually much wider?

Would this be equivalent to spinning a yo-yo above your head at one power level (representing non-dm galaxy mov't) and then adding a lot of weight to that same yo-yo (representing DM galaxy) but still spinning it at the same power level (so it now goes a lot wider and slower)?
 
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  • #2
jhe1984 said:
Okay - and this is most likely a gross oversimplification - but (for galaxies thought to have DM halos), the predicted motion/predicted spin of the galaxy would be much tighter, while the observed motion (with the DM accounted for) is actually much wider?

I'm not quite sure what you mean here. At a given distance from the center of a the galaxy, the presence of dark matter leads to a larger rotation velocity. This is because the force of gravity at that radius is stronger (more mass interior) and so a star must have more angular momentum to support a circular orbit.


Would this be equivalent to spinning a yo-yo above your head at one power level (representing non-dm galaxy mov't) and then adding a lot of weight to that same yo-yo (representing DM galaxy) but still spinning it at the same power level (so it now goes a lot wider and slower)?

Well, no, it would be sort of analogous if you left the yoyo the same size and increased the tension of the rope (to simulate the dark matter) by spinning the yoyo faster.
 
  • #3
SpaceTiger said:
I'm not quite sure what you mean here. At a given distance from the center of a the galaxy, the presence of dark matter leads to a larger rotation velocity.


Okay, so are you saying that dark matter leads only to a larger rotational velocity, but not necessarily to a different galactic shape (than if DM weren't there)?
 
  • #4
jhe1984 said:
Okay, so are you saying that dark matter leads only to a larger rotational velocity, but not necessarily to a different galactic shape (than if DM weren't there)?

It does impact the shape, but not in the way you're thinking. Dark matter is thought to be very weakly interacting, in which case it would form a roughly spherical halo. Luminous matter, however, dissipates energy very easily, so it tends to form a disk. See here for more discussion of spherical vs. disk distributions:

https://www.physicsforums.com/showthread.php?t=93098"
 
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1. What is DM halo movement?

DM halo movement refers to the motion of dark matter (DM) halos, which are large, spherical structures that are thought to surround galaxies. DM halos are made up of particles that do not interact with light, making them invisible to traditional telescopes. However, their gravitational effects can be observed on the movement of stars and gas within galaxies.

2. What is the difference between predicted and observed DM halo movement?

Predicted DM halo movement is based on theoretical models and simulations, while observed DM halo movement is based on actual measurements and observations. Predicted DM halo movement can help us understand the behavior of dark matter in different scenarios, while observed DM halo movement can provide evidence for or against these theories.

3. How is DM halo movement predicted?

DM halo movement is predicted through computer simulations and theoretical models. These models take into account factors such as the distribution of dark matter in the universe, the gravitational pull of visible matter, and the expansion of the universe.

4. How is DM halo movement observed?

DM halo movement is observed through various techniques, such as gravitational lensing, galaxy rotation curves, and the motions of stars and gas within galaxies. These observations can provide insight into the distribution and behavior of dark matter in the universe.

5. How do predicted and observed DM halo movement compare?

Predicted and observed DM halo movement can sometimes differ, as our understanding of dark matter is still incomplete. However, the more we observe and study DM halo movement, the better our predictions can become. Additionally, discrepancies between predicted and observed DM halo movement can lead to new insights and discoveries in the field of dark matter research.

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