Effects of sedimented helium on the X-ray properties of galaxy clusters
  In this Letter, we consider the role played by the sedimentation of helium nuclei on the emissivity and metallicity distribution of the X-ray emitting plasma in the cores of relaxed galaxy clusters. We model the gas density and temperature profiles of nearby cooling core clusters to estimate the gravitational acceleration acting on the helium and show that its sedimentation time scale is too long with respect to the present age of these objects to play a significant role. However, we argue that these time scales have to be definitely lower in the past allowing the helium to settle in the cluster cooling cores and raise its abundance to values higher than the solar one. A direct consequence of this speculation is that the helium, by increasing the total X-ray emissivity, reduces the measured metal abundance in the inner (r < 20 kpc) cluster regions. 
S. Ettori, A.C. Fabian