dark matter affecting galaxy rotation...

billy_boy_999

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?

mathman

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.

Nereid

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).

billy_boy_999

informative, thanks a lot guys...can i clarify something? is anomolous 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?

Nereid

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
- ...