Baryonic Mass Fraction in Rich Clusters and the Total Mass Density in the Cosmos

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Abstract
Direct observations of the supposedly universal primordial deuterium abundance imply a relatively large baryon density ΩB = (0.019-0.030) h-2 = (0.029-0.10) (95% confidence limit, hereafter CL) for Hubble constant H0 ≡ 100 h = (70 ± 15) km-1 s-1 Mpc-1. From new observations of both the X-ray bremsstrahlung and the Sunyaev-Zeldovich effect, we obtain the baryon fraction in rich clusters, fB = (0.13 ± 0.03)(62/H0)1.4. Together with the observed baryon density this implies that rich clusters are a fair sample of the universe (baryon enhancement ~ 1) and that the cosmic total mass density Ωm = (0.3-0.9) (95% CL). This conclusion agrees with dynamical measures of Ωm on large scales. Conversely, this dynamically measured Ωm > 0.3 (2.4 σ) and the rich cluster limit fB > 0.1 imply ΩB > 0.025 for h < 0.7, ~ 1. This relatively large baryon density is consistent with standard big bang nucleosynthesis and some recent 4He abundance observations.