2000A&A...355..461A

We present ROSAT/HRI and ASCA observations, combined with optical multi-object spectroscopy and photometry of A 521, a rich galaxy cluster at z=0.247. The ROSAT/HRI image of A 521 shows an irregular morphology with two peaks separated by ∼500h₅₀^–1kpc. A Wavelet analysis of the X-ray image reveals the presence of two cluster components: a diffuse main cluster, with an emission peak offset from the BCG position and a compact less massive cluster in the North West, probably in an early merger phase with the main cluster. The surface brightness profiles of both components are well fitted by two independent isothermal β-models. The overall cluster temperature derived from the ASCA spectra is 6.4±0.8keV. The density of the galaxies in the cluster central part follows an irregular, nearly cross-like, structure. Two main ridges S1 (NE/SW) and S2(NW/SE) cross south of the BCG and belong to the cluster from our velocity measurements. Whence the position of the X-Ray center of the main cluster roughly coincides with the intersection of the optical ridges S1 and S2, the segregation between the gas and the galaxies subclustering is extremely severe. This is an indication of a very particular dynamical state. Several interesting alignments are apparent. The center of the two X-ray sub-clusters are aligned with the S2 direction as well as the BCG major axis, whereas the main X-Ray cluster is elongated along the ridge axis S1. This last feature, together with the extremely large X-Ray core radius of the main cluster and the very high velocity dispersion within the ridge S1, suggests that a more ancient merger already occurred along direction S1. At larger scale, the ridges S1 and S2 point in the direction of the nearest angular neighbors among Abell clusters: A517 and (A528/A518) respectively. The observed alignment effects can be most naturally explained if A521 is a young cluster currently forming at the crossing of two filaments, one pointing towards A517 and the other in the direction of A528/A518, along which the cluster is accreting material. The intersection of filaments are thought to be the natural place of rich cluster formation and A521 might be the first observed evidence for this hypothesis.