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

TL;DR

This study investigates how electron-ion non-equipartition affects the outskirts of galaxy clusters, influencing observable properties and potentially biasing cosmological measurements, with implications for shock heating models and future radio observations.

Contribution

It provides a systematic analysis of non-equipartition effects on cluster observables and their dependence on shock heating efficiency, offering new insights into cluster physics and cosmology.

Findings

Non-equipartition causes ~10% bias in projected temperature at R_vir.

The effect on SZ bias evolves slightly with redshift, affecting cosmological measurements.

Non-equipartition effects are sensitive to electron heating efficiency and metallicity.

Abstract

(abridged) 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 \LambdaCDM cosmology using one-dimensional hydrodynamic simulations. The effects of the non-adiabatic electron heating efficiency, \beta, on the degree of non-equipartition are also studied. Using the gas fraction f_gas = 0.17 (which is the upper limit for a cluster), we give a conservative lower limit of the non-equipartition effect on clusters. Beyond the virial radius, the non-equipartition effect depends rather strongly on \beta, and such a strong dependence at the shock radius can be used to distinguish shock heating models or constrain the shock heating efficiency of electrons. We have also studied systematically the signatures of non-equipartition on X-ray and SZ observables. 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. We found that for our model in the \LambdaCDM 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.