We fit a functional form for a universal ICM entropy profile to the scaled entropy profiles of a catalogue of X-ray galaxy cluster outskirts results, which are all relaxed cool core clusters at redshift below 0.25. We also investigate the functional
form suggested by Lapi et al. and Cavaliere et al. for the behaviour of the entropy profile in the outskirts and find it to fit the data well outside 0.3r200 . We highlight the discrepancy in the entropy profile behaviour in the outskirts between observations and the numerical simulations of Burns et al., and show that the entropy profile flattening due to gas clumping calculated by Nagai & Lau is insufficient to match observations, suggesting that gas clumping alone cannot be responsible for all of the entropy profile flattening in the cluster outskirts. The entropy profiles found with Suzaku are found to be consistent with ROSAT, XMM-Newton and Planck results.
We use new Suzaku observations of PKS 0745-191 to measure the thermodynamic properties of its ICM out to and beyond r_{200} (reaching 1.25r_{200}) with better accuracy than previously achieved, owing to a more accurate and better understood backgroun
d model. We investigate and resolve the tensions between the previous Suzaku and ROSAT results for PKS 0745-191, which are found to be principally caused by incorrect background modelling in the previous Suzaku analysis. We investigate in depth the systematic errors affecting this observation, and present temperature, density, entropy and gas mass fraction profiles reaching out to and beyond the virial radius. We find that the entropy profile flattens in the outskirts as originally observed in the previous Suzaku analysis, but that the flattening starts at larger radius. The flattening of the entropy profile and our mass analysis suggests that outside ~17 (~1.9 Mpc) the ICM is out of hydrostatic equilibrium or the presence of significant non-thermal pressure support.
We present Suzaku observations of the galaxy cluster Abell 2029, which exploit Suzakus low particle background to probe the ICM to radii beyond those possible with previous observations (reaching out to the virial radius), and with better azimuthal c
overage. We find significant anisotropies in the temperature and entropy profiles, with a region of lower temperature and entropy occurring to the south east, possibly the result of accretion activity in this direction. Away from this cold feature, the thermodynamic properties are consistent with an entropy profile which rises, but less steeply than the predictions of purely gravitational hierarchical structure formation. Excess emission in the northern direction can be explained due to the overlap of the emission from the outskirts of Abell 2029 and nearby Abell 2033 (which is at slightly higher redshift). These observations suggest that the assumptions of spherical symmetry and hydrostatic equilibrium break down in the outskirts of galaxy clusters, which poses challenges for modelling cluster masses at large radii and presents opportunities for studying the formation and accretion history of clusters.
