2010A&A...514A..60S

We have discovered a strong lensing fossil group (J0454) projected near the well-studied cluster MS0451-0305. Using the large amount of available archival data, we compare J0454 to normal groups and clusters. A highly asymmetric image configuration of the strong lens enables us to study the substructure of the system. We used multicolour Subaru/Suprime-Cam and CFHT/Megaprime imaging, together with Keck spectroscopy to identify member galaxies. A VLT/FORS2 spectrum was taken to determine the redshifts of the brightest elliptical and the lensed arc. Using HST/ACS images, we determined the group's weak lensing signal and modelled the strong lens system. This is the first time that a fossil group is analysed with lensing methods. The X-ray luminosity and temperature were derived from XMM-Newton data. J0454 is located at z=0.26, with a gap of 2.5mag between the brightest and second brightest galaxies within half the virial radius. Outside a radius of 1.5Mpc, we find two filaments extending over 4Mpc, and within we identify 31 members spectroscopically and 33 via the red sequence with i<22mag. They segregate into spirals (σ_v=590km/s) and a central concentration of ellipticals (σ_v=480km/s), establishing a morphology-density relation. Weak lensing and cluster richness relations yield consistent values of r₂₀₀=810-850kpc and M₂₀₀=(0.75-0.90)x10¹⁴M_☉. The brightest group galaxy (BGG) is inconsistent with the dynamic centre of J0454. It strongly lenses a galaxy at z=2.1±0.3, and we model the lens with a pseudo-isothermal elliptical mass distribution. A high external shear, and a discrepancy between the Einstein radius and the weak lensing velocity dispersion requires that the BGG must be offset from J0454's dark halo centre by at least 90-130kpc. The X-ray halo is offset by 24±16kpc from the BGG, shows no signs of a cooling flow and can be fit by a single β-model. With L_X=(1.4±0.2)x10⁴³erg/s J0454 falls onto standard cluster scaling relations, but appears cooler (T=1.1±0.1keV) than expected (T∼2.0keV). Taken all together, these data indicate that J0454 consists of two systems, a sparse cluster and an infalling fossil group, where the latter seeds the brightest cluster galaxy. An alternative to the sparse cluster could be a filament projected along the line of sight mimicking a cluster, with galaxies streaming towards the fossil group.