Finding Out What Clusters Are Made Of
Clusters are made up of two basic types of matter: luminous matter (like stars and
hot gas) and dark matter. Dark matter does not shine on its own, and the only way
we know it exists is because of its gravitational affect on luminous matter. If we
want to know how much dark matter there is in the entire Universe, we have to
study something that is representative of the Universe as a whole. Something
big, that is. Clusters of galaxies are the largest known objects in the Universe, believed to be
big enough that they have the same fraction of dark matter as the whole Universe.
One area of cluster research is aimed at using X-ray observations to
understand how much luminous matter and dark matter exists in clusters.
Most of the luminous matter in clusters is in the form of hot gas in
galaxies. The gas, which has a temperature of 10-100 million degrees,
radiates X-rays. How much hot gas is in a cluster is simply related to the
luminosity we observe from the cluster. Thus, we can make a direct
measurement of the luminous
matter from X-ray observations of clusters of galaxies.
How much dark matter is in a cluster, however, has to be inferred from the
observable, luminous matter. We can do this because galaxy clusters are
"relaxed" systems, that is, there is a balance between the dark matter and the pressure of
the cluster. The pressure of the cluster is related to the X-ray emitting gas (which
we observe), so, by assuming an equilibrium between the two, we can estimate
the amount of dark matter.
Some Surprising Conclusions
When we study the of the amount of dark matter in clusters, we find that although
there is more dark matter than luminous matter in clusters, there is much less dark
matter than many "theories of everything" predict. One of the implications
of this, if it is true, is that the Universe would keep on expanding forever: there
would not be enough matter to halt the expansion with gravity.
Chemical Abundances in Clusters: Other Evidence
Another avenue of current research is to use the X-ray spectra to determine what kinds of elements the gas in
between the galaxies contains. These observations can then be used as a further
test of models of clusters and their evolution.
The luminous matter in a cluster is not all the same. Some of it is primordial
hydrogen and helium created in the Big Bang; some of it is heavier elements
like oxygen, neon, magnesium, silicon, and sulfur. These latter elements were
created by stars in fusion processes, or in supernova explosions. Models that can explain
how these elements got out of the stars inside the galaxies in the clusters and into
the intracluster gas might also explain why some clusters have
relatively more dark matter than others.
X-ray Missions Used in Cluster Work
Two kinds of data are needed for galaxy clusters research: high resolution images
and high-resolution spectra of the X-ray emitting gas. Currently,
the best data for scientists to use are images
from ROSAT and spectra from ASCA. New missions such as Chandra X-ray Observatory and future missions like
Constellation X will improve the quality of the data available. Scientists
will be able to include more clusters (and more distant clusters) in their
research; this will allow them to trace out the early history of clusters,
as well as the Universe as a whole.
Thank you to Michael Loewenstein for contributing to this article.