On Arcs and Omega

Original · EN

The gravitational lens effect of galaxy clusters can produce large arcs from source galaxies in their background. Typical source redshifts of 1 require clusters at z 0.3 for arcs to form efficiently. Given the cluster abundance at the present epoch, the fewer clusters exist at z 0.3 the higher Omega₀ is, because the formation epoch of galaxy clusters strongly depends on Omega₀. In addition, at fixed Omega₀, clusters are less concentrated, and hence less efficient lenses, when the cosmological constant is positive, Omegaₗambda > 0. Numerical cluster simulations show that the expected number of arcs on the sky is indeed a sensitive function of Omega₀ and Omegaₗambda. The numerical results are compatible with the statistics of observed arcs only in a universe with low matter density, Omega₀ 0.3, and zero cosmological constant. Other models fail by one or two orders of magnitude, rendering arc statistics a sensitive probe for cosmological parameters.