The Baryon Content of Groups and Clusters of Galaxies

We have analyzed the properties of a sample of 33 groups and clusters of galaxies for which both optical and X-ray data were available in the literature. This sample was built to examine the baryon content and to check for trends over a decade in temperature down to 1 keV. We examine the relative contribution of galaxies and ICM to baryons in clusters through the gas-to-stellar mass ratio (Mgas/M∗). We find that the typical stellar contribution to the baryonic mass is between 5 and 20%, at the virial radius. The ratio (Mgas/M∗) is found to be roughly independent of temperature. Therefore, we do not confirm the trend of increasing gas-to-stellar mass ratio with increasing temperature as previously claimed. We also determine the absolute values and the distribution of the baryon fraction with the density contrast δ with respect to the critical density. Virial masses are estimated from two different mass estimators: one based on the isothermal hydrostatic equation (IHE), the other based on scaling law models (SLM), the calibration being taken from numerical simulations. Comparing the two methods, we find that SLM lead to less dispersed baryon fractions over all density contrasts and that the derived mean absolute values are significantly lower than IHE mean values: at δ = 500, the baryon fractions (gas fractions) are 11.5–13.4 % (10.3–12 %) and ∼ 20% (17 %) respectively. We show that this is not due to the uncertainties on the outer slope β of the gas density profile but is rather indicating that IHE masses are less reliable. Examining the shape of the baryon fraction profiles we find that cluster baryon fractions estimated from SLM follow a scaling law. Moreover, we do not find any strong evidence of increasing baryon (gas) fraction with temperature: hotter clusters do not have a higher baryon fraction than colder ones, neither do we find the slope β to increase with temperature. The absence of clear trends between fb and Mgas/M∗ with temperature is consistent with the similarity of baryon fraction profiles and suggests that nongravitational processes such as galaxy feedback, necessary to explain the observed luminosity–temperature relaSend offprint requests to: R. Sadat tionship, do not play a dominant rôle in heating the intra-cluster gas on the virial scale.