Discovery of a redshift 0.8 cluster of galaxies in the ROSAT north ecliptic pole survey

We report the discovery of RX J1716.6+6708, an X-ray emitting cluster of galaxies at a redshift of 0.813. This cluster is only the third such X-ray emitting object known at redshifts greater than 0.8. X-ray emitting clusters at extreme distances are of special importance because they are massive, comparable to low-redshift examples, and their existence is problematic for some theories of structure formation. The luminosity of RX J1716.6+6708 is 3.2X10(44) erg s(-1) and the velocity dispersion of its member galaxies, based on 12 redshifts, is 1892 km s(-1). The galaxies in RX J1716.6+6708 are spread along a filament perhaps indicating that it is more properly called a protocluster. (C) 1997 American Astronomical Society.