ABSTRACT. We report the discovery of a quasar at z = 4.96 ± 0.03 within a few Mpc of the quasar SDSS 0338+0021 at z = 5.02 ± 0.02. The newly found quasar has SDSS i and z magnitudes of 21.2, and an estimated absolute magnitude M_B -25.2. The projected separation on the sky is 196'', and the redshift difference Δz = 0.063 ± 0.008. The probability of finding this quasar pair by chance in the absence of clustering in this particular volume is ~10^-4 to 10^-3. We conclude that the two objects probably mark a large-scale structure, possibly a protocluster, at z 5. This is the most distant such structure currently known. Our search in the field of 13 other QSOs at z 4.8 so far has not resulted in any detections of comparable luminous QSO pairs, and it is thus not yet clear how representative is this structure at z 5. However, along with the other evidence for clustering of quasars and young galaxies at somewhat lower redshifts, the observations are at least qualitatively consistent with a strong biasing of the first luminous and massive objects, in agreement with general predictions of theoretical models. More extensive searches for clustered quasars and luminous galaxies at these redshifts will provide valuable empirical constraints for our understanding of early galaxy and structure formation.

Subject headings: cosmology: observations; galaxies: formation; quasars: general; quasars: individual (SDSS 0338+0021, RD 657)

^* Based on observations obtained at the W. M. Keck Observatory, which is operated by the California Association for Research in Astronomy, a scientific partnership among California Institute of Technology, the University of California and the National Aeronautics and Space Administration.

Print publication: Issue 1 (2003 October 10)
Received 2001 September 24, accepted for publication 2003 June 16