THE TOTAL BARYON FRACTION IN GROUPS: MAPPING HOT GAS OUT TO THE VIRIAL RADIUS

Galaxy groups probably contain most of the baryons in the Universe.Unfortunately the low Xray surface brightness of groups has prevented thedetection of the gas at large radii which should dominate their baryon content.We propose, for the first time, to trace the surface brightness of a few groupsout to close to R_200. This will allow us to measure total gas and iron masses,baryon fractions, and mass/light, gas/star, and ironmass/light ratios, and toinvestigate the differences from rich clusters. Coupled with results fromexisting XMM data, this will provide template profiles for extrapolating theproperties of groups to large radii, test formation scenarios for groups, andtest whether clusters can be formed by the merger of galaxy groups. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]