/ 05 11 77 6 v 1 2 8 N ov 2 00 5 Cluster Ellipticities as a Cosmological Probe

We investigate the dependence of ellipticities of clusters of galaxies on cos-mological parameters using large-scale cosmological simulations. We determine cluster ellipticities out to redshift unity for LCDM 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 time, produce lower ellipticities. The dependence of el-lipticity on Ω m is relatively weak in the range 0.2 ≤ Ω m ≤ 0.5 for high mass clusters. The mean ellipticity ¯ e(z) decreases linearly with the amplitude of 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 el-lipticities about the mean is approximated by a Gaussian, allowing a simple characterization of the evolution of ellipticity with redshift as a function of cos-mological parameters. At z = 0, the mean ellipticity of high mass clusters is approximated by ¯ e(z = 0) = 0.248 − 0.069σ 8,0 + 0.013Ω 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. Subject headings: cosmology: theory — galaxies: clusters: general — large-scale structure of the universe