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Published September 20, 2009 | Published
Journal Article Open

Stellar and Total Baryon Mass Fractions in Groups and Clusters Since Redshift 1


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 deg^2 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 R_(500). The total sample of 118 groups and clusters with z ≤ 1 spans a range in M_(500) of ~10^(13)-10^(15) M_☉. We find that the stellar mass fraction associated with galaxies at R_(500) decreases with increasing total mass as M^(–0.37 ± 0.04)_(500), independent of redshift. Estimating the total gas mass fraction from a recently derived, high-quality scaling relation, the total baryon mass fraction (f^(stars+gas)_(500) = f^(stars)_(500) + f^(gas)_(500)) is found to increase by ~25%, when M_(500) increases from = 5 × 10^(13) M_☉ to = 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 = 5 × 10^(13) M_☉. The discrepancy decreases toward higher total masses, such that it is 1σ at = 7 × 10^(14) M_☉. We discuss this result in terms of nongravitational processes such as feedback and filamentary heating.

Additional Information

© 2009 American Astronomical Society. Received 2008 December 16, accepted for publication 2009 July 21. Published 2009 September 3. The authors thank the anonymous referee for her/his valuable comments, which led to a significant improvement of the paper. We acknowledge the contributions of the entire COSMOS collaboration; more information on the COSMOS survey are available at http://www.astr.caltech.edu/∼cosmos. This research was supported by the DFG Cluster of Excellence Origin and Structure of the Universe (http://www.universe-cluster.de). D.P. acknowledges support by the German Deutsches Zentrum für Luft- und Raumfahrt, DLR project number 50 OR 0405.

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