This works is based on a combination of extensive ROSAT surveys from a number of groups. Our work greatly owes the effort of the ROSAT team and the optical followup teams in producing data and the catalogs used in the analysis. In particular, we thank K. Mason, A. Schwope, G. Zamorani, I. Appenzeller, and I. McHardy for provoding us with and allowing us to use their data prior to publication of the catalogs. TM is supported by a fellowship from the Max-Planck-Society during his appointment at MPE. GH acknowledges DLR grants FKZ 50 OR 9403 5 and FKZ 50 OR 9908 0.
© European Southern Observatory (ESO) 2000
Online publication: December 8, 1999