X‐Ray and Sunyaev‐Zel’dovich Effect Measurements of the Gas Mass Fraction in Galaxy Clusters

Abstract

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 β-model fit jointly to the X-ray data at radii beyond 100 kpc and to all of the SZE data, (2) a nonisothermal double β-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 β-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^(+0.006)_(-0.018) and f_(gas)(SZE) = 0.116 ± 0.005^(+0.009)_(-0.027) assuming (Ω_M, Ω_Λ, 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^(+0.007)_(-0.014) and f_(gas)(SZE) = 0.121 ± 0.005^(+0.009)_(-0.016). For model 3, f_(gas)(SZE) = 0.120 ± 0.009^(+0.009)_(-0.027). 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 β-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)/(Ω_(B)/Ω_(M)) is 0.68^(+0.10)_(-0.16), 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 ΛCDM cosmology and are inconsistent with a flat matter-dominated (Ω_M = 1) universe.