2001A&A...372....1Z

We use the Veron-Cetty & Veron (2000) catalog (VV) of 13 213 quasars to investigate their possible physical grouping over angular scales 10"≤Δθ≤1000". We first 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 effects affecting heterogeneous catalogs. We find in VV a statistically significant (>3σ) excess of pairs of quasars with similar redshifts (Δz≤0.01) and angular separations in the 50"-100" range, corresponding to projected linear separations (0.2-0.5)Mpc/h₇₅ (Ω_M=1, Ω_Λ=0) or (0.4-0.7)Mpc/h₇₅ (Ω_M=0.3, Ω_Λ=0.7). There is also some excess in the 100"-600" 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 differences Δz≤0.02 and angular separations 50"≤Δθ≤100" is presented in order to stimulate further observational studies and to better understand the astrophysical and cosmological significance of these interesting objects.