The Fossil Candidate RX J1548.9+0851

Numerical simulations as well as optical and X-ray observations have shown that poor groups of galaxies can evolve to what is called a fossil group. Dynamical friction as the driving process leads to the coalescence of individual galaxies in ordinary poor groups, leaving behind nothing more than a central, massive elliptical galaxy supposed to contain the merger history of the whole group. Due to longer merging timescales for less-massive galaxies, a surrounding faint-galaxy population and an extended X-ray halo similar to that found in ordinary poor groups, is expected. Multi-object spectroscopy with VIMOS has been performed to study the faint galaxy population of the fossil candidate RX J1548.9 + 0851.