Effects of the Non-Equipartition of Electrons and Ions in the Outskirts of Relaxed Galaxy Clusters

Ka-Wah Wong, Minnesota State University Mankato
Craig L. Sarazin, University of Virginia


We have studied the effects of electron-ion non-equipartition in the outer regions of relaxed clusters for a wide range of masses in the ΛCDM cosmology using one-dimensional hydrodynamic simulations. Signatures of non-equipartition on X-ray and SZ observables are studied systematically. The effects of the non-adiabatic electron heating efficiency, β, on the degree of non-equipartition are also studied. We have calculated the effect of non-equipartition on the projected temperature and X-ray surface brightness profiles using the MEKAL emission model. The non-equipartition effect can introduce a ˜ 10% bias in the projected temperature at R_{vir} for a wide range of beta. We also found that the effect of non-equipartition on the projected temperature profiles can be enhanced by increasing metallicity. In the low energy band less than ˜ 1 keV, the non-equipartition model surface brightness can be higher than that of the equipartition model in the cluster outer regions. Future X-ray observations extending to ˜ R_{vir} or even close to the shock radius should be able to detect these non-equipartition signatures. For a given cluster, the difference between the SZ temperature decrements for the equipartition and the non-equipartition models, δΔ T_{SZE}, is larger at a higher redshift. For the most massive clusters at z ≈ 2, the differences can be δΔ T_SZE ≈ 4-5 μK near the shock radius. We also found that for our model in the ΛCDM Universe, the integrated SZ bias, Y_{non-eq}/Y_{eq}, evolves slightly (at a percentage level) with redshift, which is in contrast to the self-similar model in the Einstein-de Sitter Universe. This may introduce biases in cosmological studies using the f_{gas} technique. We discussed briefly whether the equipartition and non-equipartition models near the shock region can be distinguished by future radio observations with, for example, ALMA.