X-ray and Sunyaev-Zel'dovich effect measurements of the gas mass fraction in galaxy clusters

We present gas mass fractions of 38 massive galaxy clusters at redshifts 0.14 <= z <= 0.89, derived from Chandra X-ray and OVRO/BIMA interferometric Sunyaev-Zel'dovich effect (SZE) measurements. We use three models for the gas distribution: (1) an isothermal beta-model fit jointly to the X-ray data at radii beyond 100 kpc and to all of the SZE data, (2) a nonisothermal double beta-model in hydrostatic equilibrium with a Navarro-Frenk-White (NFW) dark matter distribution, fit jointly to all of the X-ray and SZE data, and (3) an isothermal beta-model fit only to the SZE spatial data. We show that the isothermal model well characterizes the intracluster medium outside the cluster core and provides good fits to clusters with a range of morphological properties. X-ray and SZE mean gas mass fractions for model 1 are f(gas)(X-ray) = 0.110 +/- 0.003(-0:018)(+0.006) and f(gas)(SZE) = 0.116 +/- 0.005(-0.026)(+0.009) assuming (Omega(M); Omega(Lambda); h) = (0.3; 0.7; 0.7); uncertainties are statistical followed by systematic at 68% confidence. For model 2, f(gas)(X-ray) 0.119 +/- 0.003(-0.014)(+0.007) and f(gas)(SZE) = 0.121 +/- 0.005(-0.016)(+0.009). For model 3, f(gas)(SZE) 0.120 +/- 0.009(-0.027)(+0.009). The agreement in the results shows that the core can be accounted for satisfactorily by either excluding it from fits to the X-ray data or modeling the intracluster gas with a nonisothermal double beta-model. We find that the SZE is largely insensitive to core structure. Our results indicate that the ratio of gas mass fraction within r(2500) to the cosmic baryon fraction, f(gas)/(Omega(B)/Omega(M)) , is 0.68(-0.16)(+0.10) , where statistical and systematic uncertainties are included at 68% confidence. By assuming that cluster gas mass fractions are independent of redshift, we find that the results agree with standard Lambda CDM cosmology and are inconsistent with a flat matter-dominated (Omega(M) = 1) universe.