X iv : a st ro - p h / 97 08 12 5 v 1 1 3 A ug 1 99 7 A LARGE STRUCTURE OF GALAXIES AT REDSHIFT Z ∼ 3 AND ITS COSMOLOGICAL IMPLICATIONS

We report the discovery of a highly significant concentration of galaxies at a redshift of 〈z〉 = 3.090. The structure is evident in a redshift histogram of photometrically selected “Lyman break” objects in a 9 by 18 field in which we have obtained 78 spectroscopic redshifts in the range 2.0 ≤ z ≤ 3.4. The dimensions of the structure projected on the plane of the sky are at least 11 by 8, or 14h 70 by 10h −1 70 Mpc (comoving; ΩM = 1). The concentration contains 15 galaxies and one faint (R = 21.7) QSO. We consider the structure in the context of a number of cosmological models and argue that Lyman-break galaxies must be very biased tracers of mass, with an effective bias on mass scale M ∼ 10M⊙ ranging from b ∼ 2 for ΩM = 0.2 to b > ∼ 6 for ΩM = 1. In a Cold Dark Matter scenario the large bias values suggest that individual Lyman-break galaxies are associated with dark halos of mass M ∼ 10 M⊙, reinforcing the interpretation of these objects as the progenitors of massive galaxies at Based in part on observations obtained at the W.M. Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California. Alfred P. Sloan Foundation Fellow NSF Young Investigator Alan C. Davis Fellow also Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 Hubble Fellow