AAS 197, January 2001
Session 124. Galaxy Clusters: Masses and X-ray Properties
Oral, Thursday, January 11, 2001, 1:30-3:00pm, Town and Country

## [124.04] A Complete X-ray Selected Sample of Galaxy Clusters from the ROSAT North Ecliptic Pole Survey

C. R. Mullis (Institute for Astronomy, University of Hawaii)

The ROSAT X-ray telescope performed the first all-sky survey in X-rays with an imaging detector. ROSAT scanned the sky in great circles with the longest exposure time achieved at the ecliptic poles where the scans overlap. Our region of study around the North Ecliptic Pole (NEP) is special because the data here have the largest exposure time, and consequently the highest sensitivity, of the entire dataset. The survey covers a contiguous {9\circ \times 9\circ} region centered on the NEP where there are 445 X-ray sources with fluxes measured to greater than 4\sigma significance. The median exposure time is approximately 6\,ks peaking to 37\,ks at the NEP, and the median flux limit is 7.8 \times 10\rm -14 erg s\rm -1 cm\rm -2 (0.5--2.0 keV).

We have identified the physical nature of 443 (99.6%) of these sources in a comprehensive program of imaging and spectroscopy with primarily the UH 2.2m, CFHT 3.6m, Keck-I 10m, and Keck-II 10m telescopes. The main constituents of the resulting catalog are 217 AGN (49.0%), 152 stars (34.3%), and 64 clusters & groups of galaxies (14.4%).

We use the statistically complete sample of X-ray selected galaxy clusters to investigate the nature of cluster evolution and to characterize large-scale structure. A deficit of high luminosity clusters at high redshift suggests mild negative evolution. We detect a filamentary protocluster of galaxies at high redshift and a substantial supercluster consisting of at least 21 galaxy clusters.

We gratefully acknowledge funding support from NASA, the Smithsonian Astrophysical Observatory, NATO, the ARCS Foundation and the Italian ASI-CNR.