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

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

Corresponding author: D. Sluse, dsluse@eso.org

Received: 15 November 2001
Accepted: 15 October 2002

Abstract

Burbidge et al. ([CITE]) argue that the observed number of quasar pairs with small angular separations and different redshifts (typically and ) 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.