CONSTRAINTS ON COLD DARK-MATTER THEORIES FROM OBSERVATIONS OF MASSIVE X-RAY-LUMINOUS CLUSTERS OF GALAXIES AT HIGH-REDSHIFT

During the course of a gravitational lensing survey of distant, X-ray-selected, EMSS clusters of galaxies, we have studied six X-ray-luminous (L(x) > 5 x 10(44) h(50)(-2) ergs s(-1)) clusters at redshifts exceeding z = 0.5. All of these clusters are apparently massive. In addition to their high X-ray luminosity, two of the dusters at z similar to 0.6 exhibit gravitationally lensed arcs. Furthermore, the highest redshift cluster in our sample, MS 1054-0321 at z = 0.826, is both extremely X-ray luminous (L(0.3-3.5) (keV) = 9.3 x 10(44) h(50)(-2) ergs s(-1)) and exceedingly rich with an optical richness comparable to an Abell Richness Class 4 cluster. In this Letter, we discuss the cosmological implications of the very existence of these dusters for hierarchical structure formation theories such as standard Ohm = 1 CDM, hybrid Ohm = 1 C + HDM, and flat, low-density Lambda + CDM models.