A systematic X-ray search for clusters of galaxies behind the Milky Way

We report on the design and status of the Clusters in the Zone of Avoidance (CIZA) project, the first systematic X-ray search for clusters of galaxies behind the Milky Way. We demonstrate that an X-ray survey can find galaxy clusters at low Galactic latitude where optical searches are inefficient because of massive obscuration and extinction problems. We discuss the rationale for such a survey in the context of large-scale structure studies and describe in detail the combined X-ray/optical/NIR approach used by CIZA to identify clusters of galaxies at \b\ < 20°, a region of the sky that has traditionally been excluded from earlier cluster catalogs. So far, CIZA has identified and spectroscopically confirmed 137 galaxy clusters in what used to be the zone of avoidance; additional clusters have been confirmed in imaging observations and await spectroscopic observation. We present a catalog of the 73 X-ray-brightest CIZA clusters, 53 (73%) of which are new discoveries. This sample is statistically complete out to at least z ∼ 0.075 (cz ∼ 22,500 km s(-1)). We reassess local large-scale structure in the Great Attractor (GA) region in light of this first X-ray-selected cluster sample at low Galactic latitude. Our survey discovered CIZA J1324.7-5736, a luminous cluster at l = 307.°4, b = 5.°0, cz = 5700 km s(-1), and possibly as massive as A3627, but ruled out a hypothesized X-ray-luminous cluster around the radio galaxy PKS 1343-601. We argue that this region of the GA may be best described as a wall-like, triangular structure with CIZA J1324.7-5736, A3627, and the Centaurus Cluster (A3526) at its vertices. Our survey also discovered what could be a supercluster in the same region but at cz ∼ 15,000 km s(-1), i.e., behind the GA. We speculate that the latter system may be partly responsible for the large-scale flow observed in this direction out to about that distance.