The hot gas content of fossil galaxy clusters

TL;DR

This study examines the hot gas properties of four fossil galaxy clusters using X-ray and SZ data, revealing significant radial variation in gas content and challenging the view of fossil clusters as old, undisturbed systems.

Contribution

It provides detailed thermodynamic profiles of fossil clusters combining X-ray and SZ observations, highlighting their complex gas distribution and dynamical state.

Findings

Gas mass fraction converges to cosmic baryon fraction at R500

Average concentration parameter c500 matches normal relaxed clusters

Outer regions show divergence in gas profiles, indicating clumping or hydrostatic breakdown

Abstract

We investigate the properties of the hot gas in four fossil galaxy systems detected at high significance in the Planck Sunyaev-Zeldovich (SZ) survey. XMM-Newton observations reveal overall temperatures of kT ~ 5-6 keV and yield hydrostatic masses M500,HE > 3.5 x 10e14 Msun, confirming their nature as bona fide massive clusters. We measure the thermodynamic properties of the hot gas in X-rays (out to beyond R500 in three cases) and derive their individual pressure profiles out to R ~ 2.5 R500 with the SZ data. We combine the X-ray and SZ data to measure hydrostatic mass profiles and to examine the hot gas content and its radial distribution. The average Navarro-Frenk-White (NFW) concentration parameter, c500 = 3.2 +/- 0.4, is the same as that of relaxed `normal' clusters. The gas mass fraction profiles exhibit striking variation in the inner regions, but converge to approximately the cosmic baryon fraction (corrected for depletion) at R500. Beyond R500 the gas mass fraction profiles again diverge, which we interpret as being due to a difference in gas clumping and/or a breakdown of hydrostatic equilibrium in the external regions. Overall our observations point to considerable radial variation in the hot gas content and in the gas clumping and/or hydrostatic equilibrium properties in these fossil clusters, at odds with the interpretation of their being old, evolved and undisturbed. At least some fossil objects appear to be dynamically young.