Astronomy Astrophysics

Abstract. We use the Veron-Cetty & Veron (2000) catalog (VV) of 13 213 quasars to investigate their possible physical grouping over angular scales 1000 100000. We rst estimate the number of quasar pairs that would be expected in VV assuming a random distribution for the quasar positions and taking into account observational selection eects aecting heterogeneous catalogs. We nd in VV a statistically signicant (>3) excess of pairs of quasars with similar redshifts (z 0:01) and angular separations in the 5000−10000 range, corresponding to projected linear separations (0:2−0:5) Mpc=h75(ΩM = 1;Ω = 0) or (0:4−0:7) Mpc=h75(ΩM = 0:3;Ω = 0:7). There is also some excess in the 10000−60000 range corresponding to (1−5) Mpc in projected linear separations. If most of these quasar pairs do indeed belong to large physical entities, these separations must represent the inner scales of huge mass concentrations (cf. galaxy clusters or superclusters) at high redshifts; but it is not excluded that some of the pairs may actually consist of multiple quasar images produced by gravitational lensing. Of course, a fraction of these pairs could also arise due to random projections of quasars on the sky. The list of 11 pairs of quasars with redshift dierences z 0:02 and angular separations 5000 10000 is presented in order to stimulate further observational studies and to better understand the astrophysical and cosmological signicance of these interesting objects. Key words. quasars { clusters { gravitational lenses { observations 1