Outer Regions of the Cluster Gaseous Atmospheres

Abstract

We present a systematic study of the hot gas distribution in the outer regions of regular clusters using ROSAT PSPC data. Outside the cooling flow region, the β-model describes the observed surface brightness closely, but not precisely. Between 0.3 and 1 virial radii, the profiles are characterized by a power law with slope, expressed in terms of the β-parameter, in the range β = 0.65-0.85. The values of β in this range of radii are typically larger by ≈0.05 than those derived from the global fit. There is a mild trend for the slope to increase with temperature, from 〈β〉 ≈ 0.68 for 3 keV clusters to ≈0.8 for 10 keV clusters; however, even at high temperatures there are clusters with flat gas profiles, β < 0.7. Our values of β at large radius are systematically higher, and the trend of β with temperature is weaker than was previously found; the most likely explanation is that earlier studies were affected by an incomplete exclusion of the central cooling flow regions. For our regular clusters, the gas distribution at large radii is quite close to spherically symmetric, and this is shown not to be an artifact of the sample selection. The gas density profiles are very similar when compared in units of the cluster virial radius. The radius of fixed mean gas overdensity 1000 (corresponding to the dark matter overdensity 200 for Ω = 0.2) shows a tight correlation with temperature, R ∼ T0.5, as expected from the virial theorem for clusters with the universal gas fraction. At a given temperature, the rms scatter of the gas overdensity radius is only ≈7%, which translates into a 20% scatter of the gas mass fraction, including statistical scatter due to measurement uncertainties.