On the baryon content of the Shapley Supercluster

The Shapley Supercluster is particularly rich in baryons, since it contains many X-ray luminous clusters. Using the relationship between X-ray luminosity and gas mass within |3h₅₀⁻¹| Mpc radius found for nearby clusters, it is shown that the ratio of the baryon density to the critical density in the |37\,h₅₀⁻¹| Mpc core of the supercluster |Ω b>0.18h₅₀-1.3|⁠. This is much larger than the value obtained from published calculations of cosmic nucleosynthesis, |Ω b=0.06h₅₀⁻².| The high baryon content requires that the supercluster has a total mass overdensity |ρ /ρ̄> 2.4|⁠, the largest yet established on large |(~ 35 h₅₀⁻¹ \,Mpc)| scales.