1999AJ....117.2608G

Clusters of galaxies at redshifts nearing 1 are of special importance since they may be caught at the epoch of formation. At these high redshifts there are very few known clusters. We present follow-up ASCA, ROSAT High Resolution Imager, and Keck LRIS observations of the cluster RX J1716.6+6708, which was discovered during the optical identification of X-ray sources in the north ecliptic pole region of the ROSAT All-Sky Survey. At z=0.809, RX J1716.6+6708 is the second most distant X-ray-selected cluster so far published and the only one with a large number of spectroscopically determined cluster member velocities. The optical morphology of RX J1716.6+6708 resembles an inverted S-shape filament with the X-rays coming from the midpoint of the filament. The X-ray contours have an elongated shape that roughly coincides with the weak lensing contours. The cluster has a low temperature, kT=5.66^+1.37_–0.58 keV, and a very high velocity dispersion σ_los=1522⁺²¹⁵_–150 km.s^–1. While the temperature is commensurate with its X-ray luminosity of (8.19±0.43)x10⁴⁴ h^–2₅₀ ergs.s^–1 (2-10 keV rest frame), its velocity dispersion is much higher than expected from the σ-T_X relationship of present-day clusters with comparable X-ray luminosity. RX J1716.6+6708 could be an example of a protocluster, where matter is flowing along filaments and the X-ray flux is maximum at the impact point of the colliding streams of matter.