Stellar and total baryon mass fractions in groups and clusters since redshift

Abstract

We investigate if the discrepancy between estimates of the total baryon mass fraction obtained from observations of the cosmic microwave background (CMB) and of galaxy groups/clusters persists when a large sample of groups is considered. To this purpose, 91 candidate X-ray groups/poor clusters at redshift 0.1 ≤ z ≤ 1 are selected from the COSMOS 2 deg2 survey, based only on their X-ray luminosity and extent. This sample is complemented by 27 nearby clusters with a robust, analogous determination of the total and stellar mass inside R500. The total sample of 118 groups and clusters with z ≤ 1 spans a range in M500 of ∼ 1013-1015 M⊙. We find that the stellar mass fraction associated with galaxies at R500 decreases with increasing total mass as M -0.37±0.04500, independent of redshift. Estimating the total gas mass fraction from a recently derived, high-quality scaling relation, the total baryon mass fraction (fstars+gas500 = fstars500 + fgas500) is found to increase by ∼25%, when M500 increases from 〈M〉 = 5 × 1013 M⊙ to 〈M〉 = 7 × 10 14 M⊙. After consideration of a plausible contribution due to intracluster light (11%-22% of the total stellar mass) and gas depletion through the hierarchical assembly process (10% of the gas mass), the estimated values of the total baryon mass fraction are still lower than the latest CMB measure of the same quantity (WMAP5), at a significance level of 3.3σ for groups of 〈M〉 = 5 × 1013 M ⊙. The discrepancy decreases toward higher total masses, such that it is 1σ at 〈M〉 = 7 × 1014 M ⊙. We discuss this result in terms of nongravitational processes such as feedback and filamentary heating. © 2009. The American Astronomical Society. All rights reserved.