Zw 1718 . 1 – 0108 : a highly disturbed galaxy cluster at low Galactic latitude

We report the discovery of highly distorted X-ray emission associated with the nearby cluster Zw 1718.1–0108, one of whose dominant members is the powerful radio galaxy 3C353. This cluster has been missed by previous X-ray cluster surveys because of its low Galactic latitude (b = 19.5◦), despite its brightness in the hard X-ray band (2–10 keV flux of 1.2 × 10erg cm s). Our optical CCD image of the central part of the cluster reveals many member galaxies which are dimmed substantially by heavy Galactic extinction. We have measured redshifts of three bright galaxies near the X-ray emission peak and they are all found to be around z = 0.028. The ASCA GIS and ROSAT HRI images show three aligned X-ray clumps embedded in low surface-brightness X-ray emission extended by ∼ 30 arcmin. The averaged temperature measured with ASCA is kT = 4.3± 0.2 keV, which appears to be hot for the bolometric luminosity 1.1× 10h 50 erg s, when compared to the temperatureluminosity correlation of galaxy clusters. The irregular X-ray morphology and evidence for a non-uniform temperature distribution suggest that the system is undergoing a merger of substructures. Since the sizes and luminosities of the individual clumps are consistent with those of galaxy groups, Zw 1718.1–0108 is interpreted as an on-going merger of galaxy groups in a dark matter halo forming a cluster of galaxies and thus is in a transition phase of cluster formation.