Astron. Astrophys. 320, 8-12 (1997)

1. Introduction

If QSOs have redshifts entirely of cosmological origin and are randomly distributed in space, we shall expect to find very few very close pairs with very different redshifts. The number depends on the surface density of QSOs, , and the number of fields that have been examined (N), so that the number expected by accident n is given by

where is measured in arc seconds and is the number per square degree.

Thus when the first QSO pair 1548+115A,B was discovered (Wampler et al. 1973), it was considered to be a strong argument in favor of non-cosmological QSO redshifts: its two components have separation of , and their redshifts are . The probability to find such a close pair of QSOs among the QSOs then identified was estimated to be about 1% if QSOs are distributed randomly on the sky.

In the following years the number of QSOs with measured redshifts has increased to more than 7000 (cf Hewitt & Burbidge 1993). Also the gravitational lens phenomenon has been discovered and several close pairs with identical redshifts are known (see Keeton & Kochanek 1996 for a recent compilation of gravitationally lensed QSOs and candidate systems). Added to this are a number of double QSOs with nearly identical redshifts which are likely to be genuine QSO pairs and not lensed pairs since their spectra are not identically equal (cf Schneider 1994). These pairs are usually attributed to the spatial two-point correlation between QSOs.

Comparatively recently three more very close pairs with very different redshifts have been discovered. In Section 2 we describe and discuss them and look at the probability that they are accidental configurations. In Section 3 we discuss all of the possible interpretations and implications of the results.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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