SIMBAD references

Using sensitive centimeter-wave receivers mounted on the Owens Valley Radio Observatory and Berkeley-Illinois-Maryland-Association millimeter arrays, we have obtained interferometric measurements of the Sunyaev-Zeldovich (SZ) effect toward massive galaxy clusters. We use the SZ data to determine the pressure distribution of the cluster gas and, in combination with published X-ray temperatures, to infer the gas mass and total gravitational mass of 18 clusters. The gas mass fraction, f_g, is calculated for each cluster and is extrapolated to the fiducial radius r₅₀₀ using the results of numerical simulations. The mean f_g within r₅₀₀ is 0.081^+0.009_–0.011 h^–1₁₀₀ (statistical uncertainty at 68% confidence level, assuming Ω_M=0.3, Ω_Λ=0.7). We discuss possible sources of systematic errors in the mean f_g measurement.

We derive an upper limit for Ω_M from this sample under the assumption that the mass composition of clusters within r₅₀₀ reflects the universal mass composition: Ω_Mh≤Ω_B/f_g. The gas mass fractions depend on cosmology through the angular diameter distance and the r₅₀₀ correction factors. For a flat universe (Ω_Λ≡1-Ω_M) and h=0.7, we find the measured gas mass fractions are consistent with Ω_M<0.40, at 68% confidence. Including estimates of the baryons contained in galaxies and the baryons which failed to become bound during the cluster formation process, we find Ω_M∼0.25.