Testing adiabatic contraction of dark matter in fossil group candidates
  We present deep XMM observations and ESO WFI optical imaging of two
X-ray-selected fossil group candidates, RXCJ0216.7-4749 and RXCJ2315.7-0222.
Using the X-ray data, we derive total mass profiles under the hydrostatic
equilibrium assumption. The central regions of RXCJ0216.7-4749 are found to be
dominated by an X-ray bright AGN, and although we derive a mass profile,
uncertainties are large and the constraints are significantly weakened due to
the presence of the central source. The total mass profile of RXCJ2315.7-0222
is of high quality, being measured in fifteen bins from [0.075 - 0.75]R500 and
containing three data points interior to 30 kpc, allowing comprehensive
investigation of its properties. We probe several mass models based on the
standard NFW profile or on the Sersic-like model recently suggested by
high-resolution N-body simulations. We find that the addition of a stellar
component due to the presence of the central galaxy is necessary for a good
analytical model fit. In all mass profile models fitted, the mass concentration
is not especially high compared to non-fossil systems. In addition, the
modification of the dark matter halo by adiabatic contraction slightly improves
the fit. However, our result depends critically on the choice of IMF used to
convert galaxy luminosity to mass, which leads to a degeneracy between the
central slope of the dark matter profile and the normalisation of the stellar
component. While we argue on the basis of the range of M_*/L_R ratios that
lower M_*/L_R ratios are preferred on physical grounds and that adiabatic
contraction has thus operated in this system, better theoretical and
observational convergence on this problem is needed to make further progess.
