By Silvano Molendi

At X-ray wavelengths, galaxy clusters can be divided between Cool Core systems that feature a central low temperature region, and NCC that do not. Over the last 2 decades, it has been established that major mergers frequently disrupt CC systems turning them into NCC. More recently it has also been found that the fraction of CC systems does not vary with cosmic time, strongly suggesting that CC transforming into NCC must be balanced by NCC returning to the CC state.  This can only happen if the central regions of NCC systems host some cool gas embedded within hotter plasma. Using the EPIC instrument onboard XMM-Newton we have been able to show that this may indeed be the case. Unfortunately, the limited resolution of CCDs only allows us to state that our spectra are consistent with the presence of some cool gas but not to provide direct evidence for it. Detection of emission lines from cooler gas can only be achieved through the much higher spectral resolution of micro-calorimeters that will operate onboard XRSIM and Athena (see inset Figure).  Athena/XIFU observations will allow us to detect and characterize the contribution of low temperature gas to the central regions of NCC systems thereby providing irrefutable evidence that the constant fraction of CC systems across cosmic time arises from a dynamical equilibrium where CC transforming into NCC are balanced by NCC returning to the CC state.