THE BARYON FRACTION AND VELOCITY-TEMPERATURE RELATION IN GALAXY CLUSTERS - MODELS VERSUS OBSERVATIONS

The observed baryon fraction and velocity-temperature relation in clus ters of galaxies are compared with hydrodynamic simulations in two cos mological models: standard (Omega = 1) and low-density flat (Omega = 0 .45 and lambda = 0.55) cold dark matter (CDM) models, normalized to th e COBE background fluctuations. The observed properties of clusters in clude the velocity dispersion versus temperature relation, the gas mas s versus total mass relation, and the gas mass fraction versus velocit y dispersion relation. We find that, while both cosmological models re produce well the shape of these observed functions, only low-density C DM can reproduce the observed amplitudes. We find that sigma similar t o T-0.5+/-0.1, as expected for approximate hydrostatic equilibrium wit h the cluster potential, and the ratio of gas to total mass in cluster s is approximately constant for both models. The amplitude of the rela tions, however, differs significantly between the two models. The low- density CDM model reproduces well the average observed relation of M(g as) = (0.13 +/- 0.02)M h(50)(-1.5) for clusters, while Omega = 1 CDM y ields a gas mass that is 3 times lower (M(gas) = 0.045 +/- 0.004M h(50 )(-1.5)) with both gas and total mass measured within a fiducial radiu s of 1.5 h(-1) Mpc. The cluster gas mass fraction reflects approximate ly the baryon fraction in the models, Omega(b)/Omega, with a slight an tibias. Therefore, because of the low baryon density given by nucleosy nthesis, Omega(b) similar or equal to 0.06 h(50)(-2), Omega = 1 mdoels produce too few baryons in clusters compared with observations. Scali ng our results as a function of Omega, we find that a low-density CDM model, with Omega similar to 0.3-0.4, best reproduces the observed mea n baryon fraction in clusters. The observed beta parameter of clusters , beta = sigma(2)/(kT/mu m(p)) = 0.94 +/- 0.08, discriminates less wel l between the models; it is consistent with that produced by low-densi ty CDM (1.10 +/- 0.22), while it is slightly larger than expected but still consistent with Omega = 1 (0.70 +/- 0.14).