GROUPS AND CLUSTERS OF GALAXIES
Many galaxies are clumped together into small groups or larger clusters
of galaxies which are bound together by gravitational force.
The Local Group
Our own galaxy is a member of the Local Group, shown schematically
below. Over 40 galaxies in the group are now known, but most of these are
very small compared to the Milky Way. The Local Group contains only 2 large
galaxies in addition to our own: M31 (Andromeda) and M33 (Triangulum).
The diagram on the left shows the larger members of the Local Group, and their
approximate distances from us. Of course the group is really three dimensional,
so this picture shows it in projection. On the right is a photograph of
M31, which is a little larger than our own Milky Way galaxy, and about
2 million light years away. It can be seen with the naked eye. Several of the
small dwarf galaxies which orbit M31 can also be seen in the picture.
Clusters of Galaxies
The largest clusters of galaxies contain thousands of galaxies spanning
distances of 20 million light years. These are the largest stable structures
in the Universe today. The galaxies in these clusters are moving to and fro
under the force of gravity at velocities of around 1000 km/s.
An X-ray image of a galaxy cluster looks completely different to an
optical image. Instead of individual galaxies, we see one large "blob"
of X-ray emission.
These two images, show the contrasting appearance of the Coma cluster of
galaxies - one of the closest large clusters to us - in the optical and
The X-rays we see are emitted from a sea of very hot (108 oC) gas, through
which the galaxies move. The gas is so hot because of the effects of
compression and shock waves experienced when it fell into the cluster.
The X-ray emission can be used to calculate the total mass of this
hot gas, which is invisible in optical images, and it is found to
amount to much more than the sum total of the galaxies. In fact, using
the temperature of the gas, together with its density, we can calculate
the gas pressure, and this can be used to measure the
total amount of mass present in the cluster. The result is rather
startling. The total mass of a cluster amounts to about five times
the total mass in the hot gas plus all the galaxies. In other words,
80% of the mass is in a form which does not show up in any waveband
available to astronomers. This is the so-called dark matter,
which appears to dominate the mass of the Universe. Scientists still have
little idea what this dark matter is.
The major galaxies in the Local Group are separated by large distances at
present. However the group is in the process of collapsing, and in a few
billion years our galaxy is likely to interact with M31, and may eventually
merge with it. This process can be seen in action in some
other galaxy groups at the present time.
Hubble Space Telescope image of the centre of a very compact group,
known as "Stephan's Quintet". Some of the galaxies in this group are
strongly interacting with one another at the present time, and will
probably merge into one large elliptical galaxy in the future.
Like clusters, many galaxy groups also contain hot, X-ray emitting gas.
In groups the brightness of this gas is lower than in clusters, and one
can see that X-rays are also being radiated from some of the galaxies,
especially the ellipticals , which are usually brighter
X-ray sources than the spirals .
This figure shows an optical image of a group of galaxies, with contours
representing the brightness of X-ray emission overlaid. Knots of X-ray
emission can be seen centred on many of the galaxies, but there is
also a large amount of hot gas surrounding them. As with galaxy clusters,
it is found that there is more matter in this hot (about 107 oC) gas
than in the galaxies themselves. We can also infer the presence of large
amounts of dark matter.
The growth of clusters
Astronomers believe that the Universe has developed from the "bottom-up",
with small galaxies forming first, and then merging to produce larger
ones. In the same way, galaxy groups fall together to make larger
clusters. With X-ray telescopes, it is easy to see this process in
operation, since many clusters can be caught in the act of merging. This
process takes many billions of years to complete, so we cannot watch
the merger progressing in any particular cluster. However, we can
study clusters which lie at different stages in the merger process, and
learn a great deal about physical processes involved.
XMM image of a double cluster, known as Abell 399/401, which is in the
early stages of merging. Both of these clusters are already very large
ones (each about 15 million light years across), so the final merged
cluster will be a real giant.