Astron. Astrophys. 334, L33-L36 (1998)

1. Introduction

Gravitational lensing is an important phenomenon because it probes mass distributions of galaxies independently of their optical luminosity. In particular, it provides a unique way to search for galaxies with very high mass-to-light ratios which are otherwise difficult to detect.

In a recent paper, Hawkins (1997) performs such a search, based on published data on eight two-image gravitational lens systems and candidate lens systems with image separations . Statistical arguments are presented that these are indeed genuine lens systems rather than chance associations of unrelated quasars. For example, several of the quasar pairs have more similar colours than one would expect to find in the general population of quasars, and in at least one case that the spectra are so similar that when one spectrum is divided by the other, the result is a constant ratio to within the noise (Hawkins et al. 1997). No lensing galaxies are found in six of the systems, resulting in mass-to-light ratios of up to 22000 in the most extreme case. The inference is that the lensing is done by underluminous `dark galaxies' with very substantial components of dark matter, with serious implications for cosmology as well as lensing studies. A further paper by Jimenez et al. (1997) discusses how such galaxies could be formed. However, Kochanek, Falco & Muñoz (1997) have argued against a lensing interpretation and for the hypothesis that the Hawkins lenses are binary quasar pairs, based on existing statistics of large separation lenses, because a population of wide-separation, optically-selected lenses should imply a significant, unseen, population of corresponding radio-selected lenses. They also discuss formation scenarios for such binary quasar pairs. In this paper we describe a new, well-defined, sample of gravitational lens systems selected from the JVAS/CLASS radio surveys. We discuss the search for the lensing galaxies in the 12 systems found so far and assess the implications for the proposal that dark lensing galaxies exist. In Sect. 2 we give a brief description of JVAS/CLASS and list the confirmed lens systems. In Sect. 3 we summarize the observations, both with ground-based telescopes and with the HST, in which we detect lensing galaxies in all 12 systems. Finally, in Sect. 4, we present our conclusions and discuss further the question of whether the optically-selected pairs (Hawkins 1997) are lens systems, physically unrelated quasar pairs or related quasar pairs.

© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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