Testing adiabatic contraction of dark matter in fossil group candidates

Abstract

We present deep XMM-Newton observations and ESO WFI optical imaging of two X-ray-selected fossil group candidates, RXC J0216.7-4749 and RXC J2315.7-0222. Using the X-ray data, we derive total mass profiles under the hydrostatic equilibrium assumption. The central regions of RXC J0216.7-4749 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 RXC J2315.7-0222 is of high quality, being measured in fifteen bins from [0.075−0.75] R 500 and containing three data points interior to 30 kpc, allowing comprehensive investigation of its properties. We investigate several mass models based on the standard NFW profile or on the Sérsic-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.

Fossil group origins

“…Recent observational results are often in contrast with the previous results that found no particularly high mass concentration (Democles et al 2010) and no special X-ray properties (12 fossil systems, Voevodkin et al 2010;10, Proctor et al 2011;17, Harrison et al 2012). Instead, Proctor et al (2011) claim atypical richnesses and optical luminosities, but this has not been found by Voevodkin et al (2010) and Harrison et al (2012).…”

Fossil groups origins

“…These differences could be due to an early formation epoch of FGs as suggested by simulations (Khosroshahi et al 2007). Detailed X-ray observations of some FGs also indicate that these systems were assembled at early epochs in high centrally concentrated DM halos with large mass-to-light-ratio (M/L) relations (but see also Démoclès et al 2010). Nevertheless, they do not show cooling cores as those detected in galaxy clusters, which points toward the presence of other heating mechanisms, like AGN feedback (Sun et al 2004;Khosroshahi et al 2004Khosroshahi et al , 2006Mendes de Oliveira et al 2009).…”

Planckintermediate results

“…Assuming the ICM to be a perfect gas, the thermal pressure is the genuine product of the temperature and the density, i.e., P(r) ∝ n e (r) × kT (r). Following previous X-ray works Arnaud et al 2010;Démoclès et al 2010), we used the best fit to the spectroscopic temperature profile (Démocles et al, in prep. ) with the analytical function proposed by Vikhlinin et al (2006) for each of our clusters.…”