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

Direct observations of the supposedly universal primordial deuterium abundance imply a relatively large baryon density {Omega}{sub {ital B}} = (0.019{minus}0.030) {ital h}{sup {minus}2} = (0.029{minus}0.10) (95{percent} confidence limit, hereafter CL) for Hubble constant {ital H}{sub 0} {equivalent_to} 100 {ital h} = (70 {plus_minus} 15) km{sup {minus}1} s{sup {minus}1} Mpc{sup {minus}1}. From new observations of both the X-ray bremsstrahlung and the Sunyaev-Zeldovich effect, we obtain the baryon fraction in rich clusters, {ital f}{sub {ital B}} = (0.13 {plus_minus} 0.03)(62/{ital H}{sub 0}){sup 1.4}. Together with the observed baryon density this implies that rich clusters are a fair sample of the universe (baryon enhancement {Upsilon} {approximately} 1) and that the cosmic total mass density {Omega}{sub {ital m}} = (0.3{minus}0.9) (95{percent} CL). This conclusion agrees with dynamical measures of {Omega}{sub {ital m}} on large scales. Conversely, this dynamically measured {Omega}{sub {ital m}} {gt} 0.3 (2.4 {sigma}) and the rich cluster limit {ital f}{sub {ital B}} {gt} 0.1 imply {Omega}{sub {ital B}} {gt} 0.025 for {ital h} {lt} 0.7, {Upsilon} {approximately} 1. This relatively large baryon density is consistent with standard big bang nucleosynthesis and some recent {sup 4}He abundance observations. {copyright} {ital {copyright} 1998.} {ital The American Astronomical Society}