Rotation Curve of Galaxy

Rotation Curve of Galaxy:
Dynamical studies of the Universe began in the late 1950's. This meant that instead of just looking
and classifying galaxies, astronomers began to study their internal motions (rotation for disk
galaxies) and their interactions with each other, as in clusters. The question was soon developed of
whether we were observing the mass or the light in the Universe. Most of what we see in galaxies
is starlight. So clearly, the brighter the galaxy, the more stars, therefore the more massive the
galaxy. By the early 1960's, there were indications that this was not always true, called the missing
mass problem.
The first indications that there is a significant fraction of missing matter in the Universe was from
studies of the rotation of our own Galaxy, the Milky Way. The orbital period of the Sun around the
Galaxy gives us a mean mass for the amount of material inside the Sun's orbit. But, a detailed plot
of the orbital speed of the Galaxy as a function of radius reveals the distribution of mass within the
Galaxy. The simplest type of rotation is wheel rotation shown below.

Rotation following Kepler's 3rd law is shown above as planet-like or differential rotation. Notice
that the orbital speeds falls off as you go to greater radii within the Galaxy. This is called a
Keplerian rotation curve.
To determine the rotation curve of the Galaxy, stars are not used due to interstellar extinction.
Instead, 21-cm maps of neutral hydrogen are used. When this is done, one finds that the rotation
curve of the Galaxy stays flat out to large distances, instead of falling off as in the figure above.
This means that the mass of the Galaxy increases with increasing distance from the center.

The surprising thing is there is very little visible matter beyond the Sun's orbital distance from the
center of the Galaxy. So, the rotation curve of the Galaxy indicates a great deal of mass, but there
is no light out there. In other words, the halo of our Galaxy is filled with a mysterious dark matter
of unknown composition and type.
Cluster Masses:
Most galaxies occupy groups or clusters with membership ranging from 10 to hundreds of
galaxies. Each cluster is held together by the gravity from each galaxy. The more mass, the higher
the velocities of the members, and this fact can be used to test for the presence of unseen matter.

When these measurements were performed, it was found that up to 95% of the mass in clusters is
not seen, i.e. dark. Since the physics of the motions of galaxies is so basic (pure Newtonian
physics), there is no escaping the conclusion that a majority of the matter in the Universe has not
been identified, and that the matter around us that we call `normal' is special. The question that
remains is whether dark matter is baryonic (normal) or a new substance, non-baryonic.