Transcript: It was a surprise to astronomers twenty or so years ago when clusters of galaxies began to be detected in significant numbers in x-ray emission, a surprise because astronomers did not expect to find gas in clusters of galaxies. This is because the galaxies that are dominant in clusters, elliptical galaxies, tend to have very little gas, and what gas they would have will be swept out by the rapid motion of the galaxies through the cluster gravitational potential. But it turns out that there’s a mechanism called a cooling flow by which clusters can accumulate a large amount of gas. Due to the high degree of pressure and density, the temperature of the gas elevates to several million degrees Kelvin at which point it emits in x-rays. X-rays therefore have been used to detect clusters and their hot gas out to substantial redshifts, and this is in fact one of the most effective ways of finding high redshift clusters of galaxies.