‘Is there any giant clusters even larger then galaxies cluster? What actually happened amount these clusters then?’
‘The great clusters contain only a small percentage of all galaxies. However, because of their high central concentration, they provide an attractive laboratory for study of galactic evolution, although many theories of their formation are still hypothermic.’
Gas is present in galaxy clusters, just as it is among the stars of a spiral galaxy. A variety of possible mechanisms heat this intergalactic gas. Perhaps the nearby passage of other rapidly moving galaxies induces supersonic shock waves; more probably, the heating occurred during the early history of the cluster. Once heated, the intergalactic gas becomes so diffuse that it cannot easily cool. At temperatures of hundreds of millions of degrees Kelvin, it is extremely hot and consequently emits x-radiation. Experiments have been conducted from space satellites to study the x-ray emission from rich clusters. We must go into space to study x-radiation and ultraviolet radiation because the earth’s atmosphere filters out and absorbs this short-wavelength radiation.
A
considerable amount of hot gas is found in rich clusters. It is generally the
cD-galaxy-dominated clusters that have substantial amounts of diffuse gas and
are strong x-ray emitters. There appears to be about as much diffuse matter
in these clusters as is present in visible stars. The x-ray spectrum of t
he
intergalactic gas has characteristic emission lines produced by highly ionized
iron nuclei. The nuclei are stripped of all but one or two electrons. (Compare
this structure with a terrestrial iron nucleus, surrounded by twenty-six electrons.)
The high temperature of this gas, about 10 times hotter than the center of the
sun, is responsible for the unusual stripped state of the atom. In a cooler
gas, the atomic electrons are not bombarded as frequently by collisions with
neighboring particles. Consequently they will be less easily removed, and the
atoms will not be as highly ionized.
The
abundance of iron in intergalactic gas in clusters of galaxies is not much lower
than the abundance of iron (relative to that of hydrogen) in the sun, and iron
is not the only heavy element to be detected through its x-ray emission. It
happens to produce the strongest lines in the spectral region accessible to
x-ray satellite experiments. However, in one or two nearby galaxy clusters,
other elements—notably oxygen and silicon—have also been detected via their
x-ray line emission. This result came as a considerable surprise to many astronomers,
because they had speculated that the intergalactic gas might be merely a remnant
of the primordial matter from which the galaxies condensed. 
After
all, why should the process of galaxy formation be so efficient as to exhaust
entirely the primordial gas clouds? We know, for example, that star formation
in our own galaxy is not a very efficient process. Only about 10 percent of
the mass of a dense molecular cloud will be converted into stars; the rest is
returned to the interstellar medium.
The discovery of iron in the intergalactic gas has shown that such gas cannot be completely primordial. It must somehow have been enriched by the ejection of iron, one of the end products of stellar evolution, from galaxies. The origin of this enriched intergalactic gas must have been from stars in the cluster galaxies themselves. The galaxies in large clusters are not presently shedding mass at a high enough rate for that to have happened, and so the mass loss must have occurred long ago. As galaxies age, they grow less luminous. In their early phases, many brighter and more-massive stars must have been present. These stars would have evolved rapidly. Shells of enriched matter would have been ejected as the lower-mass stars evolved into planetary nebulae and novae and the massive stars developed winds as supergiants and ultimately exploded as supernovae. This enriched gas, especially that injected by supernovae, must be the source of much of the intergalactic gas.
