The Impact of Galaxy Cluster Mergers on Cosmological Parameter Estimation from Surveys of the Sunyaev-Zel’dovich Effect

Sensitive surveys of the cosmic microwave background will detect thousands of galaxy clusters via the Sunyaev-Zel’dovich (SZ) effect. Two SZ observables, the central or maximum and integrated Comptonization parameters ymax and Y, relate in a simple way to the total cluster mass, which allows the construction of mass functions (MFs) that can be used to estimate cosmological parameters such as ΩM, σ8, and the dark energy parameter w. However, clusters form from the mergers of smaller structures, events that can disrupt the equilibrium of intracluster gas on which SZ- M relations rely. From a set of N-body/hydrodynamical simulations of binary cluster mergers, we calculate the evolution of Y and ymax over the course of merger events and find that both parameters are transiently “boosted,” primarily during the first core passage. We then use a semianalytic technique developed by Randall et al. to estimate the effect of merger boosts on the distribution functions YF and yF of Y and ymax, respectively, via cluster merger histories determined from extended Press-Schechter (PS) merger trees. We find that boosts do not induce an overall systematic effect on YFs, and the values of ΩM, σ8, and w were returned to within 2% of values expected from the nonboosted YFs. The boosted yFs are significantly biased, however, causing ΩM to be underestimated by 15%-45%, σ8 to be overestimated by 10%-25%, and w to be pushed to more negative values by 25%-45%. We confirm that the integrated SZ effect, Y, is far more robust to mergers than ymax, as previously reported by Motl et al. and similarly found for the X-ray equivalent YX, and we conclude that Y is the superior choice for constraining cosmological parameters.