Independent Evidence for Earlier Formation Epochs of Fossil Groups of Galaxies through the Intracluster Light: The Case for RX J100742.53+380046.6

Abstract Fossil groups (FG) of galaxies still present a puzzle to theories structure formation. Despite the low number bright galaxies, they have relatively high velocity dispersions and ICM temperatures often corresponding cluster-like potential wells. Their measured concentrations are typically high, indicating early formation epochs as expected from originally proposed scenario for their origin being older undisturbed systems. This is, however, in contradiction with typical lack well developed cool cores. Here, we apply cluster dynamical indicator recently discovered intracluster light fraction (ICLf) classic FG, RX J1000742.53+380046.6, assess its state. We also refine that use an independent age estimator. find negative radial temperature metal abundance gradients, achieving supersolar values at hot core. The X-ray flux concentration is consistent core ICLf analysis provides probe system’s state shows system very relaxed, more than all clusters, where same has been performed. specific about 6 times higher, any clusters previously analyzed, which noninteractive galaxy had last merging event within ∼5 Gyr. predicted be important new tool identify fossil systems constrain relative clusters.