Measurement of the rotation of the galaxy UGC 9242

by Nicole Vogt, Martha Haynes and Terry Herter using the Palomar 5m telescope. This image shows the location of the spectroscope slit on the edge-on spiral galaxy.

The spectroscope has dispersed the image vertically. You see here two emission lines. For each line you see that there is a frequency shift as you move across the image of the galaxy. This is a Doppler shift and it implies that the galaxy is rotating, one side is moving towards us, the other side is moving away. The rotation axis appears to be near the galactic centre and perp. To the plane of the galactic disc.

Here is the rotational velocity as a function of angular distance from galactic centre. Notice it NEVER DECREASES!! The galaxy has a lot of mass even out where there are not any stars. This unseen stuff is Dark Matter.