Probing dark energy via galaxy cluster outskirts

TL;DR

This paper introduces a Bayesian method combining Planck and Chandra data to analyze galaxy cluster outskirts, providing new constraints on dark energy models and the equation of state of dark energy.

Contribution

It presents a novel cosmological test using galaxy cluster outskirts and demonstrates its robustness and potential for advancing dark energy research.

Findings

Constraints favor a cosmological constant with w=-1

No evidence found for dynamic dark energy

Method is robust against various systematics

Abstract

We present a Bayesian approach to combine $Planck$ data and the X-ray physical properties of the intracluster medium in the virialization region of a sample of 320 galaxy clusters ($0.056<z<1.24$, $kT>3$ keV) observed with $Chandra$. We exploited the high-level of similarity of the emission measure in the cluster outskirts as cosmology proxy. The cosmological parameters are thus constrained assuming that the emission measure profiles at different redshift are weakly self-similar, that is their shape is universal, explicitly allowing for temperature and redshift dependence of the gas fraction. This cosmological test, in combination with $Planck$+SNIa data, allows us to put a tight constraint on the dark energy models. For a constant-$w$ model, we have $w=-1.010\pm0.030$ and $\Omega_m=0.311\pm0.014$, while for a time-evolving equation of state of dark energy $w(z)$ we have $\Omega_m=0.308\pm 0.017$, $w_0=-0.993\pm0.046$ and $w_a=-0.123\pm0.400$. Constraints on the cosmology are further improved by adding priors on the gas fraction evolution from hydrodynamic simulations. Current data favour the cosmological constant with $w\equiv-1$, with no evidence for dynamic dark energy. We checked that our method is robust towards different sources of systematics, including background modelling, outlier measurements, selection effects, inhomogeneities of the gas distribution and cosmic filaments. We also provided for the first time constraints on which definition of cluster boundary radius is more tenable, namely based on a fixed overdensity with respect to the critical density of the Universe. This novel cosmological test has the capacity to provide a generational leap forward in our understanding of the equation of state of dark energy.