First detailed X-ray observations more than 25 years ago with the Uhuru and Copernicus satellites and the first rocket borne X-ray telescopes showed that the X-ray emitting hot gas in galaxy clusters reaches high enough densities in the cluster centers that the cooling time of the gas falls below the Hubble time. The consequences of these observations have first been explored in early papers by, e.g., Silk  and Fabian and Nulsen . In the absence of a suitable fine-tuned heating source, the cooling and condensation of the gas in the central regions is a straight-forward consequence of the energy equation of the ICM. From this analysis the cooling flow scenario emerged (e.g., Fabian et al. [52, 53]). Based on the assumption of steady state cooling this model predicts spectra implying an emission distribution over a wide temperature range, a locally multi-temperature structure and as a consequence of the latter mass deposition distributed over a large fraction of the cooling flow region [89,146]. It remained a puzzle, however, why in the regions of cooling flows with estimated mass deposition rates up to several hundred or thousand solar masses per year, and little evidence for such massive cooling of gas could be found at other wavelengths (e.g. ). This is a major reason why the cooling flow model did not get accepted unanimously among astronomers. Evidence for warm gas (diffuse emission line systems), cool gas, and even star formation was well detected at lower levels in the range of 1-10% of the model predictions (1 to <100M0 yr-1) (e.g. [45, 53,72] as summarized for example in the review by McNamara ).
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