Omega baryon via Oort's Method

Abstract

The baryon density of the universe is equal to the product of the baryon-to-light ratio, M_b/L, and the luminosity density, j. We estimate M_b/L as the sum of the masses of the X-ray gas and the visible stars in a rich cluster of galaxies divided by the luminosity of the cluster galaxies in precisely the same sky aperture. We evaluate the gas-to-light ratio derived from the EMSS detect cell flux and the CNOC cluster redshift survey galaxies. After making an aperture correction to an effective overdensity of 500rho_c, we find that Omega_gas=0.012-0.016 h^-3/2, depending on the galaxy fading correction. Adding in the galaxy baryons at a mass-to-light ratio of 5 Msun/Lsun, equivalent to Omega_stars=0.003h^-1, we find that Omega_b=0.015-0.019 for H_0=100 (or 0.040-0.051 for H_0=50). Expressed as the baryon to photon ratio, eta, this corresponds to eta=4.0-5.2x10^{-10} (H_0=100) and is in the mid-range of values from other methods. The individual clusters have a dispersion about the mean Omega_{gas} of 40%, and the chi^2 of the 14 clusters is consistent with the hypothesis that the gas-to-light ratio is a universal constant. If we ignore the light of the cD, the variance increases by a factor of three. After the radial segregation of gas and light within a cluster is taken into account, these statistics indicate that there is little variation of the gas-to-light ratio from cluster to cluster over the 0.2 to 0.55 range in redshift.