The Sunyaev‐Zeldovich Effect Signature of Excess Entropy in Distant, Massive Clusters

Abstract

Studies of cluster X-ray scaling relations have led to suggestions that nongravitational processes, e.g., radiative cooling and/or "preheating," have significantly modified the entropy of the intracluster medium (ICM). For the first time, we test this hypothesis through a comparison of predicted thermal Sunyaev-Zeldovich (SZ) effect scaling relations with available data from the literature. One of the relations that we explore, in principle, depends solely on SZ effect observations, thus offering an X-ray-independent probe of the ICM. A detailed comparison of the theoretical relations with the largest compilation of high-z SZ effect data to date indicates that the presence of an entropy floor is favored by the data. Furthermore, the inferred level of that floor, K0 ≳ 300 keV cm2, is comparable to that found in studies of X-ray scaling relations of nearby massive clusters. Thus, we find no evidence for significant evolution of the entropy floor out to z ~ 0.7. We further demonstrate that the high-quality data to be obtained from the upcoming Sunyaev-Zeldovich Array (SZA) and the (soon to be) upgraded Owens Valley Radio Observatory (OVRO) array will open powerful new windows into the properties of the ICM. Specifically, the new measurements will allow for accurate measurements of the ICM entropy for even the most distant galaxy clusters.