In places where galaxies cluster together to form a group millions of light years across, the escaping hot gas between the galaxies is revealed by its X-ray emission. We have even found clues to the age of the gas. For example, in clusters where the gas has only recently emerged from its parent galaxies, it is still clumped around them, as revealed by the patchy appearance of the X-ray images. In more evolved clusters, the images are smooth, showing a diffuse, centrally-peaked distribution of hot gas. These differences in the X-ray images of clusters of galaxies are correlated with differences in the types of galaxies in volved.
Spirals seem to be associated with the patchy X-ray emission.
Thus, X-ray observations are providing basic clues to the evolution of giant systems of galaxies. We have only a few tantalizing clues just now, but the picture should become clearer after the launch of the planned Advanced X-ray Astrophysics Facility in the late 1980s.