Invisible Cluster
 The X-ray glow of million-degree gas fills the Coma Cluster of galaxies in this image from the ROSAT satellite. The gas is thicker at the center of the cluster. Some of the galaxies are visible as small red or orange points in the image. As galaxies fall toward the center of the cluster, the hot gas between galaxies strips away much of the cold gas inside the galaxies, which deprives them of the raw materials for new stars. [ROSAT/MPE/S. L. Snowden]
Some of the cluster’s “dark side” is in the form of superhot gas that glows in X-rays. All together, the gas is several times as massive as the galaxies themselves.
There’s a dynamic interplay between the hot gas and the galaxies.
As galaxies “fall” toward the center of the cluster, they fly through the hot gas, which strips away the cold gas inside the galaxies. Without their cold gas, the galaxies can’t give birth to new stars. That helps transform the appearance of some of the galaxies. Spiral galaxies lose their spiral arms, so they look like featureless disks. More about that tomorrow.
But the galaxies may have an effect on the hot gas, too. Over the eons, it should have cooled, but it hasn’t. Hot “jets” of particles from the centers of some galaxies may act like big blowtorches, keeping the gas nice and hot.
Yet even the gas and the galaxies combined make up only a small fraction of the Coma Cluster. As much as 80 percent of its mass may consist of dark matter -- a form of matter that produces no detectable energy, but that exerts a gravitational pull on the visible matter around it. The dark matter ensures that most of this impressive cluster remains invisible.
Script by Damond Benningfield, Copyright 2008
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