On the Rotation of Galaxies

Source: Rubin, Vera C., "Dark Matter in Spiral Galaxies", Scientific American, June 1963

The galaxy is NGC2998, an Sc type galaxy at a distance of 96 megaparsecs in the constellation Ursa Major. The picture of the galaxy itself shows the superimposed spectographic slit as it appeared on the telvision monitor at the four-meter Kitt Peak telescope. Below it is the hydrogen-alpha region of the spectrogram that resulted from an exposure of 200 minutes. The values are translated into the distance / velocity chart pictured. Note how one side of the galaxy shows a definate shift in the recession speed compared to the other side of the galaxy. Galactic rotation is therefore demonstrated. The formulas for deriving the rotation rate first find the rotation rate as a percentage of the speed of light and from that the actual rate given in the chart is derived. Values for this galaxy are typical of values found for other galaxies at all distances.