It's Dark Out There....
"Dark matter" refers
matter of an
unknown type that astronomers and cosmologists believe must make up the
majority of the
mass in the universe.
It is called "dark" because it does not emit any light.
We know of its presence because of the gravitational effects it has on
objects that we can see. For example, galaxies in clusters move at
speeds that are too high to be attributed just to the visible
galaxies. In addition, astronomers measure high temperature gas in
these galaxy clusters. This gas is at too high a temperature to
remain bound to the cluster without some additional, hidden, mass.
For galaxies and groups, the
X-ray data have often indicated very extended dark matter halos far beyond the
radius at which one sees starlight or galaxies. The total inferred dark
matter mass is often 10 times that in the "visible" galaxies alone.
Dark matter also plays a role in the early universe. Astronomers theorize
that the presence of dark matter helps to explain the relative amounts
of light elements and isotopes produced
in the Big Bang. Results from the
Wilkinson Microwave Anisotropy Probe (WMAP) show that 23% of the Universe is
made up of dark matter.
One of the best ways of determining the mass of a system, such as a
cluster of galaxies, group of galaxies or a massive elliptical galaxy, is to
measure the X-ray
temperature and gas profiles. Astronomers start by assuming the gas is
in equilibrium, which is borne out by the thermal spectra of the gas.
Matching models of the distribution and temperature of the gas to the
X-ray observations gives the mass of the gas. Use of this technique
has shown that clusters of galaxies are gas and
baryon rich, that is, the mass in gas exceeds the mass in
stars by
factors of 3-5 and that the total baryonic mass is ~15% of the
total mass of the cluster (i.e. visible mass plus dark matter). Since clusters are supposed to be "fair samples of the
universe" they should have a baryon fraction that corresponds to 4%, as inferred from Big Bang nucleosynthesis and results from
WMAP. However, WMAP and other evidence now point to a new component in
the universe, which is called dark energy. Dark energy makes up
73% of the energy and matter of the universe. By including this
dark energy with the visible mass and dark matter, we find that clusters really do share in the same
baryonic fraction as the rest of the universe.
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