Unveiling the Nature of Fossil Groups with XMM-HST\: Looking at the Progenitors

Even though the first fossil group (FG) was discovered almost three decades agotheir origin and evolution still remain unclear. The popular explanation thatthey are cannibalistic remains of groups that lost energy through dynamicalfriction is not consistent with their typically lack the expected large coolcores. We have raised a representative sample of FGs with high quality Xray andoptical data and developed a robust technique to estimate their relative age anddynamical stage using the ICL. The inital results have been surprising andhighlighted the need to collect data from evolutionary phases prior to FGformation (FG progenitors or FPs) to understand their origin. Here, we proposeshort XMM observations of the 2 brightest and most informative FPs. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]