The intracluster medium (ICM) contains the vast majority of the baryonic matter in galaxy clusters and is heated to X-ray radiating temperatures by collisions in their huge gravitational potential well. Matter from the warm-hot intergalactic medium can enter but not exit the clusters, which are therefore considered as quasi-closed boxes and ideal laboratories to study plasma physics and galactic evolution. X-ray spectroscopy is therefore the most important tool to obtain insights into the astrophysics of galaxy clusters. In this seminar I will recall crucial evolutionary problems in galaxy clusters and the greatest achievements obtained by X-ray spectroscopy in the recent years with XMM-Newton, Chandra, Suzaku and Hitomi X-ray satellites. I will primarily focus on the cooling flow problem: in absence of heating, the cores of a substantial fraction of galaxy clusters are expected to cool down within 1 Gyr, which is shorter than their ages. This should bring to tremendous star formations rates of several 100s solar masses per year that are not observed. Energetic feedback from the active galactic nucleus of the central, dominant, galaxy is the most commonly invoked scenario, particularly owing to its powerful radio jets. However, other phenomena such as sloshing of the ICM and shocks due to mergers are alternative, viable, means. I will show how 20 years of X-ray spectroscopy enabled transformational astrophysics and continues to provide extraordinary results from the discovery of the cooling flow problem to the search for possible solutions. I will finally go through some limitations in the current instrumentations, the unanswered new questions regarding the energy release and the need for more powerful X-ray missions in the future such as ATHENA.
The fight between cooling and heating in galaxy clusters