About one third of clusters have distinct cool cores where the gas temperature decreases inward and the radiative cooling time drops below 3 Gyr. These regions are X-ray bright and well studied. Jets from the nucleus of the central galaxy create bubbles in the hot gas which distribute heat to the surroundings, preventing it from cooling catastrophically. Such feedback stems the growth of the central galaxy. X-ray, radio, optical and infrared observations of cool core clusters and groups will be used to illustrate and identify the physics and processes which are relevant.
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