2. IMPORTANCE OF STUDIES OF THE HOT INTRAGLUSTER MEDIUM
In rich clusters the hot intracluster medium (ICM) is the dominant, luminous
component of the mass, i.e. the gas mass exceeds that in the
stellar component of the
galaxies in the cluster. Therefore, to understand clusters (and the
galaxies in them),
we must understand the origin and evolution of the gaseous component.
More specifically, studies of the ICM yield information on a rich
variety of phenomena including:
- Mapping the structure and mass distribution of clusters - Since the
emphasizes the potential wells, the X-ray images are relatively insensitive to
superpositions of low mass systems. X-ray cluster surveys show that
in optically selected clusters is very common and, therefore, that many
clusters are dynamically young.
- Role of the central galaxy - The X-ray observations of clusters
the special nature of bright centrally located galaxies (like M87 in
in Centaurus) and have suggested the presence of
"cooling flows" around central galaxies.
- Interactions of gas with radio emitting plasmas - The detection of
the ICM and
detailed maps have permitted studies related to confinement and morphology of
radio emitting plasmas (e.g., WAT and head-tail sources).
- Measurement of H0 through the Sunyaev-Zel'dovich Effect
- Combination of imaging
and spectroscopy of the X-ray emitting plasma with radio measurements of
the microwave decrement have provided estimates of H0 and
suggest that the next
generation of X-ray observatories (e.g., ASTRO-D, BBXRT, and
AXAF) will have
adequate sensitivity to yield precise (10-20%) estimates of the value of
the Hubble constant for clusters with redshifts up to z ~ 0.5.
- Trace large scale structure - Clusters are luminous X-ray sources
and are readily
identifiable to large redshift (with moderate angular resolution)
because of their
large physical sizes. Therefore, they can be used to trace the large
scale distribution of matter.
- Study evolution of clusters with redshift - One of the
the X-ray properties of clusters (e.g., substructure and X-ray
luminosity) is the
gravitational collapse of the cluster system. Therefore, X-ray cluster
different epochs can provide information on the initial perturbations
from which large scale structures form.
- Determine the properties of the X-ray emitting gas at various
epochs. The X-ray
emitting gas in clusters has been found to contain heavy elements which
be produced through nucleosynthesis in stellar systems. Thus, studies of
can tell us about galaxy evolution, the IMF of galaxies, the formation
galaxies in clusters (and groups), and the mass loss history of
The above list of topics shows the rich variety of problems which can be
through studies of the ICM. The X-ray study of clusters has become
that it is no longer possible to review all of the material in a single
hence, this contribution emphasizes four topics - the dynamical
evolution of clusters
(and the role of the central galaxy), substructure in clusters, the
origin of the ICM,
and concludes with a brief review of what we can expect to learn from
future X-ray astronomy missions.