A LARGE STRUCTURE OF GALAXIES AT REDSHIFT Z-SIMILAR-TO-3 AND ITS COSMOLOGICAL IMPLICATIONS

We report the discovery of a highly significant concentration of galax ies at a redshift of [z] = 3.090. The structure is evident in a redshi ft histogram of photometrically selected ''Lyman-break'' objects in a 9' by 18' field in which we have obtained 78 spectroscopic redshifts i n the range 2.0 less than or equal to z less than or equal to 3.4. The dimensions of the structure projected on the plane of the sky are at least 11' by 8', or 14 h(70)(-1) by 10 h(70)(-1) Mpc (comoving; Omega( M) = 1). The concentration contains 15 galaxies and one faint (R = 21. 7) QSO. We consider the structure in the context of a number of cosmol ogical models and argue that Lyman-break galaxies must be very biased tracers of mass, with an effective bias on mass scale M similar to 10( 15) M. ranging from b similar to 2 for Omega(M) = 0.2 to b greater tha n or similar to 6 for Omega(M) = 1. In a cold dark matter scenario, th e large bias values suggest that individual Lyman-break galaxies are a ssociated with dark halos of mass hi similar to 10(12) M., reinforcing the interpretation of these objects as the progenitors of massive gal axies at the present epoch. Preliminary results of spectroscopy in add itional fields suggest that such large structures are common at z simi lar to 3, with about one similar structure per survey held. The implie d space density is consistent with the possibility that we are observi ng moderately rich clusters of galaxies in their early nonlinear evolu tion. Finally, the spectrum of one of the QSOs discovered in our surve y (z(em) = 3.356) exhibits metal-line absorption systems within the th ree redshift bins having the largest number of galaxies in field, z = 2.93, 3.09, and 3.28. These results are the first from an ongoing ''ta rgeted'' redshift survey designed to explore the nature and distributi on of star-forming galaxies in the redshift range 2.7 less than or sim ilar to z less than or similar to 3.4.