X-ray exploration of the outskirts of the nearby Centaurus cluster using Suzaku and Chandra

Abstract

We present Suzaku observations of the Centaurus cluster out to 0.95r200, taken along a strip to the north west. We have also used congruent Chandra observations of the outskirts to resolve point sources down to a threshold flux around 7 times lower than that achievable with just Suzaku data, considerably reducing the systematic uncertainties in the cosmic X-ray background emission in the outskirts. We find that the temperature decreases by a factor of 2 from the peak temperature to the outskirts. The entropy profile demonstrates a central excess (within 0.5r200) over the baseline entropy profile predicted by simulations of purely gravitational hierarchical structure formation. In the outskirts the entropy profile is in reasonable agreement with the baseline entropy profile from Voit et al., but lies slightly below it. We find that the pressure profile agrees with the universal pressure profile of Arnaud et al. but lies slightly above it in the outskirts. The excess pressure and decrement in entropy in the outskirts appear to be the result of an excess in the measured gas density, possible due to gas clumping biasing the density measurements high. The gas mass fraction rises and reaches the mean cosmic baryon fraction at the largest radius studied. The clumping corrected gas mass fraction agrees with the expected hot gas fraction and with the simulations of Young et al. We further the analysis of Walker et al. which studied the shapes of the entropy profiles of the clusters so far explored in the outskirts with Suzaku. When scaled by the self similar entropy the Suzaku entropy profiles demonstrate a central excess over the baseline entropy profile, and are consistent with it at around r500 . However outside r500 the entropy profiles tend to lie below the baseline entropy profile.