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ngc_4414.jpgGalaxies typically contain about 100 billion stars.  Nearby, 99% of all galaxies are spirals or ellipticals.  The spiral galaxies have prominent spiral arms which are defined by young, hot, blue stars and regions of star formation.   The elliptical galaxies are featureless and appear to be mostly devoid of young stars.

In spiral galaxies the characteristic spiral arms lie in a flattened, rotating disc held together by the overall gravitation field of the galaxy.  In elliptical galaxies, which are also held together by gravity, the stars have much higher random velocities, with a substantial non-circular component.

In many galaxies, there must be a large amount of dark matter present.  The evidence for this comes from the rotation curves of the discs of the spirals, that is, the variation in the speed of rotation of the stars and gas of the galaxy with the distance from its centre.  In many cases, the rotation curves are “flat” as far as they and be measured.  This type of rotation curve means that the mass of the galaxy must continue to increase to large distances, despite the fact that the light falls off very rapidly.  Consequently there must be dark matter in the outer regions of galaxies.

The nature of dark matter is one of the great unsolved problems in cosmology.  It is no longer thought to consist of dead stars or brown dwarfs but may be some form of as yet undiscovered elementary particle.

Comparing the speeds of rotation of matter in the galaxy with the light emitted provides evidence for dark matter.

Colliding Galaxies

antennae.jpgA few galaxies cannot be classified as spiral or elliptical.  When two galaxies collide, their gravity pulls them into very strange shapes.  The system known as the Antennae was formed when two spiral galaxies passed by each other in the same sense as the rotation of each disc of stars, that is, in the “prograde” direction.  The stars in the outer rinds felt the same outward force acting in the same direction for a prolonged period, and so were ripped off.

Since nearly all galaxies occur in clusters, similar encounters between galaxies are quite common on a less spectacular scale.  Within the Local Group of galaxies, the Large and Small Magellanic Clouds are currently being torn apart by the gravitational influence of our galaxy and eventually some of this debris will be assimilated by our galaxy, the Milky Way.

In rich clusters of galaxies, massive galaxies can form at their centres, due to the collision and coalescence of smaller galaxies.
Collisions are important in explaining the formation and evolution of galaxies.

Clusters of Galaxies

Clusters of galaxies are the largest gravitationally bound systems in the universe.  The largest clusters contain thousands of galaxies.  Just like the stars in elliptical galaxies, the random velocities of the galaxies in the cluster balance the attractive gravitational force of the cluster as a whole.  In addition to galaxies, clusters of galaxies possess hot intergalactic gas and large amounts of dark matter.

Hubble Space Telescope image of the cluster Cl0024 + 1654, showing the multiple images of a distant blue galaxy lensed by the cluster. (HST)

Evidence for hot gas in clusters come from X-ray observations.  The gravitation pull of the total mass of the cluster is so strong that the gas has to be very hot indeed to form a stable atmosphere within the cluster.

The temperature of the hot gas in a massive cluster is typically about 10 million degrees.   It turns out that there is as much mass in the hot diffuse intergalactic gas as there is in the galaxies themselves.  However, the total mass of the cluster is about 10 times the observable mass of the galaxies and the hot intergalactic gas, so there must be large amounts of dark matter present.

The distribution of the hot gas enable s the distribution of the dark matter in the cluster to be determined.  X-ray observations have suggested that the distribution of the dark matter is similar to that of the galaxies and the hot gas.

comacluster.jpgAnother remarkable aspect of the gravitational influence of clusters of galaxies is that they act as giant gravitational lenses, which can magnify and distort the images of distant background galaxies.  This enables the mass of the cluster to be determined, and also the observation of the distant background galaxies, which would otherwise be too faint to be detected.

X-ray image of the Coma Cluster in the 0.5-2.4 keV energy band observed by the ROSAT observatory.