Page "Galaxy groups and clusters" Paragraph 14

Clusters of galaxies are the most recent and most massive objects to have arisen in the hierarchical structure formation of the universe and the study of clusters tells one about the way galaxies form and evolve.

Clusters have two important properties: their masses are large enough to retain any energetic gas ejected from member galaxies and the thermal energy of the gas within the cluster is observable within the X-Ray bandpass.

The observed state of gas within a cluster is determined by a combination of shock heating during accretion, radiative cooling, and thermal feedback triggered by that cooling.

The density, temperature, and substructure of the intracluster X-Ray gas therefore represents the entire thermal history of cluster formation.

To better understand this thermal history one needs to study the entropy of the gas because entropy is the quantity most directly changed by increasing or decreasing the thermal energy of intracluster gas.