Helium abundance (and $H_0$) in X-COP galaxy clusters
We present the constraints on the helium abundance in 12 X-ray luminous galaxy clusters that have been mapped in their X-ray and Sunyaev-Zeldovich (SZ) signals out to $R_{200}$ for the XMM-Newton Cluster Outskirts Project (X-COP). The unprecedented precision available for the estimate of $H_0$ allows us to investigate how much the reconstructed X-ray and SZ signals are consistent with the expected ratio $x$ between helium and proton densities of 0.08-0.1. We find that a $H_0$ around 70 km/s/Mpc is preferred from our measurements, with lower values of $H_0$ as requested from the Planck collaboration (67 km/s/Mpc) requiring a 34% higher value of $x$. On the other hand, higher values of $H_0$, as obtained by measurements in the local universe, impose $x$, from the primordial nucleosynthesis calculations and current solar abundances, reduced by 37--44\%.