XMM Temperature Measurements of ''Dark.. Groups of Galaxies

We propose observations of two dark.. groups of galaxies to determine ifthey represent a distinct class of objects. Also known as OLEGS or fossilgroups.., they exhibit Lx, total mass, and gas mass similar to poor clusters,but optically they contain an isolated elliptical with no significantcompanions. A striking common feature is an abnormally high total mass to lightratio (virgul5001000 for some objects!). These results are however uncertain, and athorough investigation of these objects is needed. We have selected from theliterature 2 particularly intriguing systems which have not been wellstudied.Their comparison with normal. groups of similar mass and Lx will provide vitalinformation on the process of galaxy/group formation. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]