Joint Suzaku and Chandra observations of the MKW4 galaxy group out to the virial radius

We present joint Suzaku and Chandra observations of MKW4. With a global temperature of 1.6 keV, MKW4 is one of the smallest galaxy groups that have been mapped in X-rays out to the virial radius. We measure its gas properties from its centre to the virial radius in the north, east, and north-east directions. Its entropy profile follows a power law of ∝r^1.1 between R₅₀₀ and R₂₀₀ in all directions, as expected from the purely gravitational structure formation model. The well-behaved entropy profiles at the outskirts of MKW4 disfavour the presence of gas clumping or thermal non-equilibrium between ions and electrons in this system. We measure an enclosed baryon fraction of 11 per cent at R₂₀₀, remarkably smaller than the cosmic baryon fraction of 15 per cent. We note that the enclosed gas fractions at R₂₀₀ are systematically smaller for groups than for clusters from existing studies in the literature. The low baryon fraction of galaxy groups, such as MKW4, suggests that their shallower gravitational potential well may make them more vulnerable to baryon losses due to active galactic nucleus feedback or galactic winds. We find that the azimuthal scatter of various gas properties at the outskirts of MKW4 is significantly lower than in other systems, suggesting that MKW4 is a spherically symmetric and highly relaxed system.