The Influence of Mergers on Scatter and Evolution in Sunyaev–zel’dovich Effect Scaling Relations

The Sunyaev–Zel'dovich effect (SZE) observable-mass (Y–M) scaling relation is a promising technique for obtaining mass estimates for large samples of galaxy clusters and holds a key to studying the nature of dark matter and dark energy. However, cosmological inference based on SZE cluster surveys is limited by our incomplete knowledge of scatter, and evolution in the Y–M relation. In this work, we investigate the effects of galaxy cluster mergers on the scaling relation using the Omega500 high-resolution cosmological hydrodynamic simulation. We show that the non-thermal pressure associated with merger-induced gas motions contributes significantly to the scatter, and evolution of the scaling relation. After the merger, the kinetic energy of merging systems is slowly converted into thermal energy through dissipation of turbulent gas motions, which causes the thermal SZE signal to increase steadily with time. This post-merger evolution is one of the primary source of scatter in the Y–M relation. However, we show that when the missing non-thermal energy is accounted for, the resulting relation exhibits little to no redshift evolution and the scatter around the scaling relation is ∼20%–30% smaller than that of the thermal SZE signal alone. Our work opens up a possibility to further improve the current robust mass proxy, Y, by accounting for the missing non-thermal pressure component. We discuss the future prospect of measuring internal gas motions in galaxy clusters and its implication for cluster-based cosmological tests.