Dark matter affecting galaxy rotation

In summary, galaxies rotate around their centers at a slower speed than solar systems. This is due to the mass present in the galaxy. If you can measure the speed of a star around the center of a galaxy, you can determine the mass of the galaxy. If there is more mass in a galaxy than can be accounted for by the 'light' then it is likely that there is dark matter in the galaxy.
  • #1
billy_boy_999
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i don't understand how dark matter acts upon the rotation of a galaxy. galaxy rotation doesn't work the same way that solar system rotation does in that it's not a simple gradient of further planets rotate slower, right? nor is it further planets rotate faster, like a vinyl record...i take it our galaxy's rotation is a tad more complex, can anyone explain more lucidly the problems we have run into with trying to figure out the gravitational workings of our galaxy and how dark matter enters into the equation?
 
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  • #2
Qualitatively, the essential point is that stars are revolving around galactic centers at a speed which would lead to them leaving the galaxy if the only matter there was ordinary matter (visible or otherwise). That is, more mass is needed to hold the galaxy together against centrifugal force.
 
  • #3
With some important caveats, the rate at which a star rotates about the centre of the galaxy it's in depends only on the total mass 'interior' (between the star and the centre of the galaxy) to the star. Therefore, if you can measure the speed with which stars are moving around the centre of a galaxy, as a function of distance from the centre, you can determine the mass of the galaxy, as a function of that same distance.

That's what 'galaxy rotation rates' is shorthand for; you put the slit of your spectroscope along the observed plane of the galaxy (making sure you centre it on the galaxy nucleus), then the deviation of the spectal lines (e.g. HII, [OII], Ca II) from the centre will measure this rotation function. (there are other ways to get it too).

We think we know how to estimate the total amount of 'normal' mass from the observed total 'light' (I'm simplifying a lot). When you plug these numbers in, you find an astonishing thing - there seems to be more mass in a galaxy (as you go further out from the nucleus) than you can account for from the 'light' ... and it gets worse the further out you go.

Oh, and it's not just 'cause we only looked at a dozen or three galaxies; it seems to be just about every galaxy (some notable exceptions).
 
  • #4
informative, thanks a lot guys...can i clarify something? is anomalous galactic rotation the ONLY evidence we have of the existence of dark matter? that is, if tomorrow we found out that galactic rotation is dependent on some kind of gravity degredation, like maybe gravitons disperse over large distances or something farfetched, would that completely dissolve the dark matter question?
 
  • #5
Originally posted by billy_boy_999
informative, thanks a lot guys...can i clarify something? is anomalous galactic rotation the ONLY evidence we have of the existence of dark matter? that is, if tomorrow we found out that galactic rotation is dependent on some kind of gravity degredation, like maybe gravitons disperse over large distances or something farfetched, would that completely dissolve the dark matter question?
Please read the long sticky thread in GA&C, it's all about exactly this question.

In a nutshell:
- gravitation lensing
- virial theorem applied to cluster velocities
- IGM equilibrium pressure and temperature
- LSS theories
- ...
 

1. What is dark matter?

Dark matter is a hypothetical type of matter that does not emit or absorb light, and therefore cannot be seen directly. It is believed to make up about 85% of the total matter in the universe and is thought to play a crucial role in the formation and evolution of galaxies.

2. How does dark matter affect galaxy rotation?

Dark matter has a gravitational pull on visible matter, causing it to rotate around the center of the galaxy. Without the presence of dark matter, galaxies would not have enough mass to maintain their observed rotation speeds.

3. How do scientists study the effects of dark matter on galaxy rotation?

Scientists use various methods, such as measuring the rotation speeds of stars and gas in galaxies, gravitational lensing, and computer simulations, to study the effects of dark matter on galaxy rotation.

4. Can dark matter be detected?

Dark matter cannot be directly detected, as it does not interact with light. However, its presence can be inferred through its gravitational effects on visible matter.

5. What are some potential implications of dark matter affecting galaxy rotation?

The presence and behavior of dark matter have significant implications for our understanding of the universe, including the formation of galaxies, the evolution of the universe, and the nature of gravity. Studying dark matter can also provide insights into the fundamental properties of matter and the origins of the universe.

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