Detecting rotation speed of Galaxies

[mr.spaceman]

The dark matter concept arose when astronomers measured rotation speed of the galaxies and they deduced that there must be more matter than we can see. And then appeared solution Dark matter.

 

I'm curious how they can measure the rotation speed of the galaxies since in terms of a human lifetime its rotation is very slow. For example our solar system completes its orbit every 225 million years or so. Our rotation speed (220 km/s I don't know how acurate is this data) is fast but Galaxies is very large and I cannot figure out how can you detect its rotation through telescope since you cannot see moving stars unless very close ones to the centre.

[swansont]

I expect it's from the Doppler shift measured in the emission (and maybe absorption) spectra. The part of the galaxy moving toward you will be blue-shifted relative to the part moving away, which will be red-shifted. All superimposed on the overall red/blue shift from the linear movement of the galaxy. It is likely that the rotational shifts just show up as a broadening/blurring of the line, since you should get a continuous distribution of speeds from different locations in the galaxy, which probably can't be spatially resolved, for galaxies too far away.

[mr.spaceman]

Thank you for answer

I don't know why the Doppler shift didn't come to my mind