J0454-0309: evidence of a strong lensing fossil group falling into a poor galaxy cluster
Abstract
Aims: 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.
Methods: 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.
Results: J0454 is located at z = 0.26, with a gap of 2.5 mag between the brightest and second brightest galaxies within half the virial radius. Outside a radius of 1.5 Mpc, we find two filaments extending over 4 Mpc, and within we identify 31 members spectroscopically and 33 via the red sequence with i < 22 mag. They segregate into spirals (σ_v = 590 km s-1) and a central concentration of ellipticals (σ_v = 480 km s-1), establishing a morphology-density relation. Weak lensing and cluster richness relations yield consistent values of r200 = 810-850 kpc and M200 = (0.75-0.90) × 1014 M_⊙. 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 90-130 kpc. The X-ray halo is offset by 24 ± 16 kpc from the BGG, shows no signs of a cooling flow and can be fit by a single β-model. With LX = (1.4 ± 0.2) × 1043 erg s-1 J0454 falls onto standard cluster scaling relations, but appears cooler (T = 1.1 ± 0.1 keV) than expected (T ~ 2.0 keV). 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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- May 2010
- DOI:
- 10.1051/0004-6361/200913810
- arXiv:
- arXiv:1002.1080
- Bibcode:
- 2010A&A...514A..60S
- Keywords:
-
- galaxies: evolution;
- galaxies: clusters: individual: J0454-0309;
- galaxies: formation;
- Astrophysics - Cosmology and Extragalactic Astrophysics
- E-Print:
- 20 pages, 17 figures