Groups and Clusters of Galaxies

 Clusters of galaxies are the most massive entities in our Universe that are held together by the force of gravity. In addition to galaxies, clusters contain vast amounts of hot gas at temperatures in excess of 10 million Kelvin. When observed at optical wavelengths, the galaxies dominate the picture like in this image of the rich cluster A2218 obtained with the Hubble Space Telescope:

(Note the multitude of gravitational arcs in this image; these are images of distant background galaxies distorted by the gravitational field of the cluster in the foreground.)

At X-ray wavelengths, the galaxies are hardly visible as the emission is dominated by bremsstrahlung radiation from the hot intra-cluster gas. Here is an image of the same cluster as before (A2218) as obtained with the High Resolution Imager aboard the ROSAT satellite:

(This X-ray image is shown at logarithmic intensity scale; also the raw image was adaptively smoothed to enhance low-surface brightness features. More examples of the power of adaptive smoothing can be found on my IDL page.)

This X-ray image covers a region more than twice as large as the one shown in the HST image above. No individual galaxies are seen - only a huge, slightly asymmetric blob of hot gas. This complementarity of optical and X-ray observations of galaxy clusters is nicely summarized
 on this picture: