Quasar pairs with arcminute angular separations

Free access article

Issue		A&A Volume 372, Number 1, June II 2001


Page(s)		1 - 7
Section		Cosmology
DOI		http://dx.doi.org/10.1051/0004-6361:20010283

A&A 372, 1-7 (2001)
DOI: 10.1051/0004-6361:20010283

Quasar pairs with arcminute angular separations

V. I. Zhdanov^{1, 2} and J. Surdej¹

¹ Institut d'Astrophysique, Université de Liège, Avenue de Cointe 5, 4000 Liège, Belgium
² Astronomical Observatory of Kyiv University, Observatorna St. 3, UA- 04053 Kyiv, Ukraine

(Received 19 October 2000 / Accepted 16 February 2001)

Abstract
We use the Véron-Cetty & Véron (2000) catalog (VV) of 13 213 quasars to investigate their possible physical grouping over angular scales $10\arcsec\leq\Delta\theta\leq 1000\arcsec$ . 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 (>3 $\sigma$ ) excess of pairs of quasars with similar redshifts ( $\Delta z\leq 0.01$ ) and angular separations in the $50\arcsec{-}100\arcsec$ range, corresponding to projected linear separations $(0.2{-}0.5) {\rm Mpc}/h_{75} (\Omega_{\rm M}=1, \Omega_\Lambda=0)$ or $(0.4{-}0.7) {\rm Mpc}/h_{75} (\Omega_{\rm M}=0.3, \Omega_\Lambda = 0.7)$ . There is also some excess in the $100\arcsec{-}600\arcsec$ 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 $\Delta z\leq 0.02$ and angular separations $50\arcsec\leq \Delta\theta\leq 100\arcsec$ is presented in order to stimulate further observational studies and to better understand the astrophysical and cosmological significance of these interesting objects.

Key words: quasars -- clusters -- gravitational lenses -- observations

Offprint request: J. Surdej, surdej@astro.ulg.ac.be

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