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 Tew instead of on the spectroscopic X-ray temperature Tspec, which is obtained by actual observational data. In a recent paper, Mazzotta et al. show that if the intracluster medium is thermally complex, Tew fails at reproducing Tspec. They propose a new formula, the spectroscopic-like temperature, Tsl, which approximates Tspec better than a few percent. By analyzing a set of hydrodynamical simulations of galaxy clusters, we find that Tsl is lower than Tew by 20%-30%. As a consequence, the normalization of the M-Tsl 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 β-model gas density profile, as is often done for observed clusters, then the M-T relation decreases by about 40% and significantly reduces its scatter. On the basis of this result, we conclude that using the observed M-T relation to infer the amplitude of the power spectrum from the X-ray temperature function could bias low σ8 by 10%-20%. This may alleviate the tension between the value of σ8 inferred from the cluster number density and those from the cosmic microwave background and large-scale structure.