Cluster Ellipticities as a Cosmological Probe

Abstract

We investigate the dependence of ellipticities of clusters of galaxies on cosmological 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 ellipticity on $Ω_m$ is relatively weak in the range $0.2 \leq Ω_m \leq 0.5$ for high mass clusters. The mean ellipticity $\bar{e}(z)$ 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 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 $\bar{e}(z=0) = 0.248-0.069 σ_{8,0} + 0.013 Ω_{m,0}$. 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, $σ_{8,0}$.