We quantify the importance of the mechanical energy released by radio-galaxies inside galaxy groups. We use scaling relations to estimate the mechanical energy released by 16 radio-AGN located inside X-ray detected galaxy groups in the COSMOS field.
By comparing this energy output to the host groups gravitational binding energy, we find that radio galaxies produce sufficient energy to unbind a significant fraction of the intra-group medium. This unbinding effect is negligible in massive galaxy clusters with deeper potential wells. Our results correctly reproduce the breaking of self-similarity observed in the scaling relation between entropy and temperature for galaxy groups.
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 pu
rpose, 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_sun. We find that the stellar mass fraction associated with galaxies at R_500 decreases with increasing total mass as (M_500)^-0.37 pm 0.04, independent of redshift. Estimating the total gas mass fraction from a recently derived, high quality scaling relation, the total baryon mass fraction (f_500^stars+gas=f_500^stars+f_500^gas) is found to increase by ~ 25% when M_500 increases from <M>=5 X 10^13 M_sun to <M> = 7 X 10^14 M_sun. After consideration of a plausible contribution due to intra--cluster 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 3.3sigma for groups of <M>=5 X 10^13~M_sun. The discrepancy decreases towards higher total masses, such that it is 1sigma at <M>= 7 X 10^14~M_sun. We discuss this result in terms of non--gravitational processes such as feedback and filamentary heating.
