What component do X-ray Telescopes observe in galaxy clusters?

The hot intracluster medium (ICM)

X-ray telescopes are indispensable tools for studying galaxy clusters because they specifically target the emission from the intracluster medium (ICM). The ICM is a diffuse plasma filling the space between galaxies, heated to extreme temperatures, often reaching tens of millions of degrees Celsius due to the cluster's immense gravitational potential. At these temperatures, the gas glows intensely across the X-ray spectrum. Observing the distribution and temperature of this X-ray emission allows researchers to map the cluster's overall gravitational potential well, providing a crucial measurement that confirms the presence of the non-luminous mass required to gravitationally confine such hot gas.