Galaxy "rotation curves"


In the 1970's it was discovered that stars in spiral galaxies rotate around the center in a very unusual way.  The stars in the outer parts of the galaxies seem to be moving too fast for the galaxy to hold them on circular orbits - they ought to fly off into intergalactic space. Since galaxies are common and stable enough looking things,  it seems unlikely that we would be lucky enough to be getting a look at them through our telescopes moments before they fly apart and dissolve into the depths of space.

This simple observation has been followed up in a great many ways over the last 30 years. The conclusion most people draw is that what we see is not all that's out there: galaxies contain much more matter than we can see directly.  This wouldn't be such a big deal if the amount of this so called dark matter were not so disturbingly large - many studies indicate that most of the matter in the Universe is so dark as to be so far invisible to our telescopes.

We think there is a lot of matter out there even if we cannot see it.  As much as 90 percent of the Universe might be dark; indicating that, in effect, we currently do not know what the Universe is made of!

  MW rotation curve
The main evidence for dark matter comes from rotation curves of disk galaxies. A rotation curve measures the rate at which stars and/or gas move in their circular orbits around the center of the galaxy. In general the curves are "flat", that is, the stars and gas rotate at the same speed from the inner parts out to the edge. This is not at all what one expects from he visible matter in the galaxies --- the outer parts of the galaxy should move slower than the inner parts, like the planets do in the Solar System. At Tuorla Observatory,
Chris Flynn and  Rami Rekola are examing faint spiral galaxies to see if they also contain dark matter.


On the left, a spiral galaxy image, with spiral arms delineated by so-called HII regions. On the right, the light from a narrow strip running along the major axis of the galaxy has been spread into a spectrum, between about 6500 and 6800 Angstroms. The rotation of the galaxy is seen in the emission lines from H alpha at 6563 Angstroms (the brightest line), as well as other fainter lines in this region due to Nitrogen. The "S" shaped curve allows the rotation of the galaxy to be measured.