Mismatch between X-ray and emission-weighted temperatures in galaxy clusters: Cosmological implications

The thermal properties of hydrodynamical simulations of galaxy clusters are usually compared to observations by relying on the emission-weighted temperature T-ew instead of on the spectroscopic X-ray temperature T-spec, which is obtained by actual observational data. In a recent paper, Mazzotta et al. show that if the intracluster medium is thermally complex, T-ew fails at reproducing T-spec. They propose a new formula, the spectroscopic-like temperature, T-sl, which approximates T-spec better than a few percent. By analyzing a set of hydrodynamical simulations of galaxy clusters, we find that T-sl is lower than T-ew by 20%-30%. as a consequence, the normalization of the M-T-sl relation from the simulations is larger than the observed one by about 50%. If masses in simulated clusters are estimated by following the same assumptions of hydrostatic equilibrium and beta-model gas density profile, as is often done for observed clusters, then the M-T relation decreases by about 40% and significantly amplitude of the power spectrum from the X-ray temperature function could bias low sigma(s) by 10%-20%. This may alleviate the tension between the value of sigma(s) inferred from the cluster number density and those from the cosmic microwave background and large-scale structure.