Authors: Ka-Wah Wong (University of Alabama), Craig Sarazin (University of Virginia), Ji Li (MIT Kavli Institute for Astrophysics and Space Research) and Daniel Wik (NASA GSFC)

The densities in the outer regions of clusters of galaxies are very low, and the collisional timescales are very long. As a result, electrons and ions may not achieve equipartition and heavy elements will be under-ionized after they have passed through the accretion shock. We have studied systematically the effects of non-equilibrium ionization for relaxed clusters in the LambdaCDM cosmology using one-dimensional hydrodynamic simulations. We found that the most prominent non-equilibrium ionization signature is the O VII and O VIII line ratio. The ratios for non-equilibrium ionization and collisional ionization equilibrium models are different by more than an order of magnitude at radii beyond half of the shock radius. These non-equilibrium ionization signatures are equally strong for models with different non-adiabatic shock electron heating efficiencies. We have also calculated the detectability of the O VII and O VIII lines with the future International X-ray Observatory (IXO). Depending on the line ratio measured, we conclude that an exposure of ∼130–380 ks on a moderate-redshift, massive regular cluster with the X-ray Microcalorimeter Spectrometer (XMS) on the IXO will be sufficient to provide a strong test for the non-equilibrium ionization model. Finally, we briefly discuss that the non-equipartition effects can bias the cosmological parameter estimation from SZ surveys.