The baryon content of groups and clusters of galaxies

We have analyzed the properties of a sample of 33 groups and clusters of ga laxies for which both optical and Xray data were available in the literatur e. This sample was built to examine the baryon content and to check for tre nds over a decade in temperature down to 1 keV. We examine the relative contribution of galaxies and ICM to baryons in clus ters through the gas-to-stellar mass ratio (M-gas/M*). We find that the typ ical stellar contribution to the baryonic mass is between 5 and 20%, at the virial radius. The ratio (M-gas/M*) is found to be roughly independent of temperature. Therefore, we do not confirm the trend of increasing gas-to-st ellar mass ratio with increasing temperature as previously claimed. We also determine the absolute values and the distribution of the baryon fr action with the density contrast delta 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 l aw models (SLM), the calibration being taken from numerical simulations. Co mparing the two methods, we find that SLM lead to less dispersed baryon fra ctions over all density contrasts and that the derived mean absolute values are significantly lower than IHE mean values: at delta = 500, the baryon f ractions (gas fractions) are 11.5-13.4% (10.3-12%) and similar to 20% (17%) respectively. We show that this is not due to the uncertainties on the out er slope beta of the gas density profile but is rather indicating that IHE masses are less reliable. Examining the shape of the baryon fraction profil es, we find that cluster baryon fractions estimated from SLM follow a scali ng 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 beta to increase w ith temperature. The absence of clear trends between f(b) and M-gas/M* with temperature is c onsistent with the similarity of baryon fraction profiles and suggests that non-gravitational processes such as galaxy feedback, necessary to explain the observed luminosity-temperature relationship, do not play a dominant ro le in heating the intra-cluster gas on the virial scale.