The Intracluster Plasma: A Universal Pressure Profile?

The pressure profiles of the intracluster plasma in galaxy clusters show a wide variance when observed in X-rays at low redshifts, z <~ 0.2. We find the profiles to follow two main patterns, featuring either a steep or a shallow shape throughout both core and outskirts. We trace these shapes back to a physical dichotomy of clusters into two classes, marked by either low entropy (LE) or high entropy (HE) throughout. From X-ray observations and Sunyaev-Zel'dovich (SZ) stacked data at higher redshifts 0.2 <~ z <~ 0.4, we obtain evidence of an increasing abundance of HEs relative to LEs. We propose this to constitute a systematic trend toward high z; specifically, we predict the pressure profiles to converge into a truly universal HE-like template for z >~ 0.5. We submit our physical templates and converging trend for further observational tests, in view of current and upcoming measurements of individual, stacked, and integrated SZ signals.