EDP Sciences
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Volume 397, Number 2, January II 2003
Page(s) 539 - 544
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20021524

A&A 397, 539-544 (2003)
DOI: 10.1051/0004-6361:20021524

Close pairs of quasars with different redshifts: New observations and results

D. Sluse1, 2, J. Surdej1, J.-F. Claeskens1, Y. De Rop1, D. W. Lee1, A. Iovino3 and M. R. S. Hawkins4

1  Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août, 17, B5C, 4000 Sart Tilman, Belgium
2  European Southern Observatory, Alonso de Cordova 3107, Santiago 19, Chile
3  Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
4  Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, Scotland, UK

(Received 15 November 2001 / Accepted 15 October 2002 )

Burbidge et al. (1997) argue that the observed number of quasar pairs with small angular separations and different redshifts (typically $\Delta \theta \leq 5\,\arcsec$ and $\Delta z > 0.1 $) is not compatible with a random distribution of quasars over the sky. After a brief review of all known quasar pairs with different redshifts, we show by means of very simple calculations that the probability of finding the three accepted pairs accidentally is of the order of 10%. We conclude that, under realistic hypotheses, the observed number of quasar pairs with different redshifts is not unlikely. We also present arguments showing that gravitational lensing biases are probably not strong enough to significantly increase the expected number of quasar pairs. The failure to detect with HST a secondary lensed image of the background quasar near the foreground one in these three pairs supports this view.

Key words: quasars: general -- gravitational lensing

Offprint request: D. Sluse, dsluse@eso.org

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© ESO 2003