Modeling the global and structural properties of the intracluster medium
Ian G. McCarthy, Michael L. Balogh, Arif Babul, Gregory B. Poole, D. Horner · Swinburne figshare (Swinburne University of Technology) · 2004
Theoretical studies of galaxy clusters that consider only gravitationally-driven processes fail to account for their observed global as well as structural X-ray properties. This has led to increased interest in models in which either radiative cooling or entropy injection (and/or redistribution) play a central role in mediating the thermal and spatial properties of the intracluster medium. Both sets of models predict the slope and the normalization of the mean luminosity-temperature (L-T) and luminosity-mass (L-M) in good agreement with the observations. Radiative cooling alone, however, results in fractions of cold/cooled baryons in excess of observationally established limits. And, the simplest entropy injection models, by design, do not treat the ``cooling core'' structure present in many clusters and therefore, cannot account for structural features like declining entropy profiles towards cluster centres revealed by recent high resolution X-ray observations. We consider models that marry radiative cooling with entropy injection, and confront model predictions for the global and structural properties of massive clusters with the latest X-ray data. The models successfully and simultaneously reproduce the observed L-T and L-M relations, yield detailed entropy and temperature profiles in excellent agreement with observations, and predict cooled gas fraction that is consistent with observational constraints. More interestingly and importantly, the model provides a possible explanation for the significant intrinsic scatter present in the L-T and L-M relations. Understanding the origin of this scatter is crucial if clusters are to be used as probes for precision cosmology studies, such as the determination of σ_8. Our model also offers a natural way of distinguishing between clusters classically identified as ``cooling flow'' clusters and the relaxed ``non-cooling flow'' clusters. The former correspond to systems that experienced only mild levels (≲ 300 keV cm^2) of entropy injection while the latter are identified as systems that suffered much higher entropy injection. The dividing line in entropy injection between the two categories corresponds roughly to the cooling threshold for massive clusters. This finding suggests that entropy injection may be an important, if not the primary, factor in determining which class a particular cluster will belong to. These results also suggest that the previously identified relationship between inferred cooling flow strength and the dispersion in the L-T relation is a manifestation of the distribution of entropy injections levels experienced by clusters. This is borne out by the entropy profiles derived from Chandra and XMM-Newton.