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 cluster is thermally complex, Tew fails at reproducing Tspec, and propose a new formula, the spectroscopic-like temperature, Tsl, which approximates Tspec better than a few per cent. By analyzing a set of hydrodynamical simulations of galaxy clusters, we find that Tsl is lower than Tew by 20–30 per cent. As a consequence, the normalization of the M–Tsl relation from the simulations is larger than the observed one by about 50 per cent. If masses in simulated clusters are estimated by following the same assumptions of hydrostatic equilibrium and β–model gas density profile, as often done for observed clusters, then the M–T relation decreases by about 40 per cent, and significantly reduces its scatter. Based on 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 per cent. This may alleviate the tension between the value of σ8 inferred from the cluster number density and those from cosmic microwave background and large scale structure. Subject headings: galaxies: clusters: general – X-rays: galaxies masses – cosmology: observations