Lessons learned from 19 years of high‐resolution X‐ray spectroscopy of galaxy clusters with the reflection grating spectrometer on board XMM‐Newton

The intracluster medium (ICM) contains the vast majority of the baryonic matter in galaxy clusters and is heated to X-ray radiating temperatures. X-ray spectroscopy is therefore a key to understanding both the morphology and the dynamics of galaxy clusters. Here we recall crucial evolutionary problems of galaxy clusters unveiled by 19 years of high-resolution X-ray spectroscopy with the reflection grating spectrometer (RGS) on board XMM-Newton. Its exquisite combination of effective area, spectral resolution, and excellent performance over two decades has enabled transformational science and important discoveries such as the lack of strong cooling flows, the constraints on ICM turbulence, and cooling-heating balance. The ability of RGS to resolve individual ICM spectral lines reveals in great detail the chemical enrichment in clusters by supernovae and asymptotic giant branch stars. RGS spectra clearly show that the ICM plasma is overall in thermal equilibrium, which is unexpected given the wealth of energetic phenomena such as jets from supermassive black holes and mergers.