The Baryon Content of Groups and Clusters of Galaxies

Abstract

We have analyzed the properties of a sample of 33 groups and clusters of galaxies and examine the baryon content 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 and find that the typical stellar contribution to the baryonic mass is between 5 and 20%. This ratio 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 distribution of the baryon fraction with the density contrast. 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). Comparing the two methods, we find that SLM lead to less dispersed baryon fractions over all density contrasts and that the derived mean absolute valuesare significantly lower than IHE mean values. 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, neither do we find the slope $β$ to increase with temperature. The absence of clear trends between $f_{b}$ and $M_{gas}/M_{*}$ with temperature is consistent with the similarity of baryon fraction profiles and suggests that non-gravitational processes such as galaxy feedback, do not play a dominant rôle in heating the intra-cluster gas on the virial scale.