RXJ1716.6+6708: a protocluster at z=0.81?

Abstract

At z=0.809, RXJ1716.6+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 RXJ1716.6+6708 resembles an inverted S-shape filament with the X-rays coming from the midpoint of the filament. The ROSAT HRI contours have an elongated shape that roughly coincide with the weak lensing contours. ASCA measures a low temperature, kT=5.7 keV. Keck-II LRIS spectra indicate a very high velocity dispersion, of sigma(v)=1522 km/s. While the temperature is commensurate with its X-ray luminosity, its velocity dispersion is much higher than expected from the sigma(v)-T(X) relationship of present-day clusters with comparable X-ray luminosity. RXJ1716.6+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.