A Census of Baryons in Galaxy Clusters and Groups

We determine the contribution of stars in galaxies, intracluster stars, and the intracluster medium to the total baryon budget in nearby galaxy clusters and groups. We find that the baryon mass fraction (fb ≡ Ωb/Ωm) within r500 is constant for systems with M 500 between 6 × 10 13 M⊙ and 1 × 10 M⊙. Although fb is lower than the WMAP value, the shortfall is on the order of both the observational systematic uncertainties and the depletion of baryons within r500 that is predicted by simulations. The data therefore provide no compelling evidence for undetected baryonic components, particularly any that would be expected to vary in importance with cluster mass. A unique feature of the current analysis is direct inclusion of the contribution of intracluster light (ICL) in the baryon budget. With the addition of the ICL to the stellar mass in galaxies, the increase in X-ray gas mass fraction with increasing total mass is entirely accounted for by a decrease in the total stellar mass fraction, supporting the argument that the behavior of both the stellar and X-ray gas components is dominated by a decrease in star formation efficiency in more massive environments. Within just the stellar component, the fraction of the total stellar luminosity in the central, giant brightest cluster galaxy (BCG) and ICL (hereafter the BCG+ICL component) decreases as velocity dispersion (σ) increases for systems with 145 km s ≤ σ ≤ 1026 km s, suggesting that the BCG+ICL component, and in particular the dominant ICL component, grows less efficiently in higher mass environments. The degree to which this behavior arises from our sample selection, which favored systems with central, giant elliptical galaxies, remains unclear. A more robust result is the identification of low mass groups with large BCG+ICL components, demonstrating that the creation of “intracluster” stars does not require a massive cluster environment. Within r500 and r200, the BCG+ICL contributes on average 40% and 33% of the total stellar light, respectively, for the clusters and groups in our sample. Because these fractions are functions of both enclosed radius and system mass, care should be exercised when comparing these values with other studies and simulations. Subject headings: galaxies: clusters: general — galaxies:cD, formation, evolution, fundamental parameters