A DARK CLUSTER OF GALAXIES AT REDSHIFT Z=1

The abundance of metals in the hot, gaseous X-ray haloes of galaxy clu sters depends crucially on the evolution of the constituent galaxies a nd their associated stellar populations. The metal abundances in X-ray clusters at high redshifts should therefore provide important insight s into the nature and epoch of galaxy formation. Here we report the de tection of an extended X-ray source in the direction of the lensed qua si-stellar object MG2016 + 112 (refs 1, 2). Although deep optical sear ches have failed to reveal a galaxy duster at the lens position(3,4), the X-ray emission is consistent with thermal bremsstrahlung radiation from a hot metal-rich, diffuse gaseous halo, as observed in nearby ga laxy dusters. This is the most distant galaxy duster discovered in X-r ays so far. Furthermore, the mass of the duster derived from this emis sion is consistent with that implied by lensing models of the system(5 ). Given that the duster apparently comprises few galaxies, yet contai ns a large amount of iron, a new type of astronomical object is implie d by our results. A revision of theoretical models of the metal enrich ment process in galaxy dusters may therefore be required.