Substructure and Scatter in the Mass-Temperature Relations of Simulated Clusters

Galaxy clusters exhibit regular scaling relations among their bulk properties. These relations establish vital links between halo mass and cluster observables. Precision cosmology studies that depend on these links benefit from a better understanding of scatter in the mass-observable scaling relations. Here, we study the role of merger processes in introducing scatter into the M − T_X relation, using a sample of 121 galaxy clusters simulated with radiative cooling and supernova feedback, along with three statistics previously proposed to measure X-ray surface brightness substructure. These are the centroid variation (w), the axial ratio (η), and the power ratios (P₂₀ and P₃₀). We find that in this set of simulated clusters, each substructure measure is correlated with a cluster's departures δ ln T_X and δ ln M from the mean M − T_X relation, both for emission-weighted temperatures T_EW and for spectroscopic-like temperatures T_SL, in the sense that clusters with more substructure tend to be cooler at a given halo mass. In all cases, a three-parameter fit to the M − T_X relation that includes substructure information has less scatter than a two-parameter fit to the basic M − T_X relation.