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H. Ebeling, C.R. Mullis, R.B. Tully (Institute for Astronomy)
X-ray selected clusters of galaxies represent sensitive and unbiased probes of the distribution of structure at the largest spatial and mass scales. In the context of large-scale structure studies cluster surveys complement earlier and ongoing work using (field) galaxies as tracers. Although a great improvement over the small statistical samples of only a decade ago, the latest X-ray cluster samples compiled from ROSAT All-Sky Survey (RASS) data still exclude the historical `zone of avoidance' at |b|<20 deg defined much earlier in the optical waveband. This gap in the all-sky map of galaxy clusters seriously hampers studies of large scale structure and local streaming motions.
Taking advantage of the fact that hard X-rays can penetrate the plane where extinction and obscuration render optical cluster searches inefficient, we are now closing the gap by selecting cluster candidates from the set of spectrally hard RASS sources at low Galactic latitude. Candidates confirmed in CCD imaging observations (at R and/or K) are scheduled for spectroscopic observations.
So far, we have spectroscopically confirmed 101 clusters at |b|<20 deg; 80% of these were previously unknown. Here we describe our target selection and cluster identification procedure, present the CIZA sample (for `Clusters In the Zone of Avoidance'), and discuss the implications of our findings for the distribution of large scale structure behind the Galactic plane, with special emphasis on the Great Attractor region.