Cluster number density normalization from the observed mass--temperature relation

Abstract

The number density of rich clusters in the local Universe is currently believed to provide the most robust normalization of power spectrum at a scale of 10 Mpc. This normalization depends very sensitively on the calibration between virial mass M and temperature T, which is usually taken from simulations. Uncertainties in the modelling, such as gas cooling and heating, can lead to a factor of 2 variations in the normalization and are thus not very reliable. Here we use instead an empirical M500—T relation derived from X-ray mass determinations to calibrate the method. We use results from dark matter simulations to relate the virial mass function to the mass function at observed M500. Using the relation of Finoguenov et al. we find that the best-fitting value in flat models is σ8= (0.77 ± 0.07) (Ωm/0.3)−0.44(Γ/0.2)0.08, where only the statistical error is quoted. This is significantly lower than previously obtained values from the local cluster abundance. A lower value for σ8 is in a better agreement with cosmic microwave background and large-scale structure constraints and helps alleviate small-scale problems of cold dark matter models. Presently the systematic uncertainties in the mass determination are still large, but ultimately this method should provide a more reliable way to normalize the M—T relation. This can be achieved by obtaining a larger sample of well-measured cluster masses out to a significant fraction of the virial radius with BeppoSAX, Chandra and XMM—Newton.