Cluster Ellipticities as a Cosmological Probe

We investigate the dependence of the ellipticities of clusters of galaxies on cosmological parameters using large-scale cosmological simulations. We determine cluster ellipticities out to redshift unity for λCDM models with different mean densities Ω_m and amplitudes of mass fluctuation σ_8,0. The mean ellipticity increases monotonically with redshift for all models. Larger values of σ_8,0, i.e., earlier cluster formation times, produce lower ellipticities. The dependence of ellipticity on Ω_m is relatively weak in the range 0.2 ≤ Ω_m ≤ 0.5 for high-mass clusters. The mean ellipticity (z) decreases linearly with the amplitude of mass fluctuations at the cluster redshift z, nearly independent of Ω_m; on average, older clusters are more relaxed and are thus less elliptical. The distribution of ellipticities about the mean is approximated by a Gaussian, allowing a simple characterization of the evolution of ellipticity with redshift as a function of cosmological parameters. At z = 0, the mean ellipticity of high-mass clusters is approximated by (z = 0) = 0.245 - 0.070σ_8,0 + 0.020Ω_m,0. This relation opens up the possibility that, when compared with future observations of large cluster samples, the mean cluster ellipticity and its evolution could be used as a new, independent tool to constrain cosmological parameters, especially the amplitude of mass fluctuations, σ_8,0.