Galaxy Sans Dark Matter

ChromaticDragon writes "Astronomers have crunched some numbers on a galaxy to discover that its rotation can be fully explained by the gravity of the observable matter — in effect, this galaxy seems to lack dark matter. This shouldn't come as a total surprise given that one of the stronger observations of Dark Matter was the Bullet Cluster where supposedly a good deal of Dark Matter and good old fashion regular matter had separated."

Comments

A good link

[The Empiricist]

I'm not sure if it is the story the submitter was trying to link to, but this article seems to cover the subject.

Re:Impact on gravity theories

[syousef]

I'm not the GP but can answer your question.

You measure the dopler shift of the stars on each side of the galaxy. Waves from stars travelling towards you compress, waves travelling away from you so it helps if galaxy is seen more edge on than top or bottom towards us.

(Of course it won't be perfectly edge on so you have to calculate the component that is edge on to work out actual speeds around the galactic center. The less edge on the more accurate you can be because the component that's edge on is larger). ...which leads to how do you measure doppler shifts.

One way is to look at the spectral lines of light in a star (ie split the light through a prism or diffraction grating). Chemicals that make up the star's surface absorb at precisely known wavelengths. It's actually really easy to do some calculation once you know what wavelength these lines have shifted to. (I did it when I did my astronomy masters. It's basic algebra andsimple equations). The difficult part is building equipment that can measure spectra so accurately. In the early days they'd be literally measuring the difference between wavelengths on glass plates.

http://aether.lbl.gov/www/science/galrotcurve.html
"To make a rotation curve one calculates the rotational velocity of stars along the length of a galaxy by measuring their Doppler shifts, and then plots this quantity versus their respective distance away from the galactic center."