The galaxy cluster outskirts probed by Chandra

We studied the physical properties of the intracluster medium (ICM) in the virialization region of a sample of 320 clusters (0.056 < z < 1.24, kT ≳ 3 keV) in the Chandra archive. With the emission measure profiles from this large sample, the typical gas density, gas slope and gas fraction can be constrained out to and beyond R₂₀₀. We observe a steepening of the density profiles beyond R₅₀₀ with β ∼ 0.68 at R₅₀₀ and β ∼ 1 at R₂₀₀ and beyond. By tracking the direction of the cosmic filaments approximately with the ICM eccentricity, we report that galaxy clusters deviate from spherical symmetry, with only small differences between relaxed and disturbed systems. We also did not find evolution of the gas density with redshift, confirming its self-similar evolution. The value of the baryon fraction reaches the cosmic value at R₂₀₀; however, systematics due to non-thermal pressure support and clumpiness might enhance the measured gas fraction, leading to an actual deficit of the baryon budget with respect to the primordial value. This study has important implications for understanding the ICM physics in the outskirts.