Mass and Metallicity of Five X-Ray-bright Galaxy Groups

Abstract

We present ASCA X-ray observations of a sample of Ðve groups selected from a cross-correlation of the ROSAT All-Sky Survey with the White et al. optical catalog of groups. These X-rayÈbright groups significantly increase the number of known systems with temperatures between 2 and 3 keV. They have element abundances of roughly 0.3x, which are typical of clusters, but their favored ratio of Si-to-Fe abundance is lower than the cluster value. Combining the ASCA results with ROSAT imaging data, we calculate total masses of a few to several times 1013 M gas mass fractions of D10%, and baryonic _, mass fractions of at least 15%È20% within a radius of 0.5 Mpc. Upper limits for the ratios of gas to galaxy mass and of the iron mass to galaxy luminosity overlap with the range observed in rich clusters and extend to lower values, but not to such low values as seen in much poorer groups. These results support the idea that groups, unlike clusters, are subject to the loss of their primordial and processed gas and show that this transition occurs at the mass scale of the 2È3 keV groups. A discussion of ASCA calibration issues and a comparison of ROSAT and ASCA temperatures are included in an Appendix.