Galaxy cluster gas fractions from interferometric measurements of the Sunyaev-Zel'dovich effect
Interferometric measurements of the Sunyaev-Zel'dovich effect toward 18 highly x-ray luminous galaxy clusters are presented. The observations were made using centimeter-wave receivers specifically constructed for these observations. The data were taken with the receivers mounted on the Owens Va...
| Summary: | Interferometric measurements of the Sunyaev-Zel'dovich effect toward 18 highly x-ray luminous galaxy clusters are presented. The observations were made using centimeter-wave receivers specifically constructed for these observations. The data were taken with the receivers mounted on the Owens Valley Radio Observatory and Berkeley-Illinois-Maryland Association millimeter arrays between 1994 and 1998. The interferometric data are used to determine the gas mass fraction in these clusters in a uniform method. The inteferometric data contain sufficient spatial information to derive models for the pressure distribution of the cluster gas. From these models, under the assumption that the gas is isothermal, the cluster gas masses are estimated and the total gravitating masses are inferred. The total gravitating mass measurement requires the additional assumption that the gas is in hydrostatic equilibrium with the cluster potential. The cluster gas temperatures are obtained from x-ray spectral observations or, in the few cases in which spectra were unavailable, estimated from x-ray luminosity-temperature relations in the literature. Since the experiment best measures the gas fraction within a fixed angular radius, the measured gas fractions are extrapolated to a fiducial radius, using a relation derived from published numerical simulations, to facilitate comparisons. The best estimate of the cluster gas fraction at τ_(500), the radius at which the enclosed mean density is 500 times the critical density, is (0.071^(+0.010)_(-0.012))h^(-1)_(100) at 68% confidence. Under the assumption that clusters are fair samples of the universe, the mass composition in clusters at the virial radius should reflect the universal mass composition. The intracluster gas is the dominant component of a cluster's baryonic mass, and so the gas mass fraction is a good approximation of the baryon mass fraction. The baryon fraction in clusters, f_B, together with the universal baryon mass density, Ω_B, sets a limit on the universal mass density, in the standard cosmological paradigm: Ω_M = Ω_B/ f_B. The cluster gas fraction measurements presented here set an upper limit to the universal mass density: Ω_Mh_(100) ≤ 0.27^(+0.07)_(-0.06), at 68% confidence. We make our best estimate of the universal matter density, including with the gas fraction estimates of the baryonic mass contained in galaxies and the baryonic mass which failed to become bound during the cluster formation process: Ω_M = 0.29^(+0.08)_(-0.06) at 68% confidence for h = 0.7. |
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