Superclustering at Redshift Z = 0:54

We present strong evidence for the existence of a supercluster at a redshift of z = 0:54 in the direction of Selected Area 68. From the distribution of galaxies with spectroscopic redshifts we nd that there is a large over-density of galaxies (a factor of four over the number expected in an unclustered universe) within the redshift range 0:530 < z < 0:555. By considering the spatial distribution of galaxies within this redshift range (using spectroscopic and photometric redshifts) we show that the galaxies in SA68 form a linear structure passing from the South-West of the survey eld through to the North-East (with a position angle of approximately 35 East of North). This position angle is coincident with the positions of the X-ray clusters CL0016+16, RX J0018.3+1618 and a new X-ray cluster, RX J0018.8+1602, centered near the radio source 54W084. All three of these sources are at a redshift of z 0:54 and have position angles, derived from their X-ray photon distributions, consistent with that measured for the supercluster. Assuming a redshift of 0.54 for the distribution of galaxies and a FWHM dispersion in redshift of 0.020 this represents a coherent structure with a radial extent of 31 h Mpc, transverse dimension of 12 h Mpc, and a thickness of 4 h Mpc. The detection of this possible supercluster demonstrates the power of using X-ray observations, combined with multicolor observations, to map the large scale distribution of galaxies at intermediate redshifts. Subject headings: galaxies: distances and redshifts | large-scale structure of universe | techniques: photometric