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 $M$-$T_{\rm X}$ 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 ($\eta$), and the power ratios ($P_{20}$ and $P_{30}$). We find that in this set of simulated clusters, each substructure measure is correlated with a cluster's departures $\delta \ln T_{\rm X}$ and $\delta \ln M$ from the mean $M$-$T_{\rm X}$ relation, both for emission-weighted temperatures $T_{\rm EW}$ and for spectroscopic-like temperatures $T_{\rm SL}$, 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 $M$-$T_{\rm X}$ relation that includes substructure information has less scatter than a two-parameter fit to the basic $M$-$T_{\rm X}$ relation.