Cluster Ellipticities as a Cosmological Probe

Abstract

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 Ωm and amplitudes of mass fluctuation σ8,0. The mean ellipticity increases monotonically with redshift for all models. Larger values of σ8,0, i.e., earlier cluster formation times, produce lower ellipticities. The dependence of ellipticity on Ωm is relatively weak in the range 0.2 ≤ Ωm ≤ 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 Ωm; 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 - 0.070σ8,0 + 0.020Ωm,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, σ8,0.