AND ASTROPHYSICS Letter to the Editor Lensing galaxies: light or dark?

Abstract. In a recent paper, Hawkins (1997) argues on the ba-sis of statistical studies of double-image gravitational lenses and lens candidates that a large population of dark lenses exists and that these outnumber galaxies with more normal mass-to-light ratios by a factor of 3:1. If correct, this is a very important re-sult for many areas of astronomy including galaxy formation and cosmology. In this paper we discuss our new radio-selected gravitational lens sample, JVAS/CLASS, in order to test and constrain this proposition. We have obtained ground-based and HST images of all multiple-image lens systems in our sam-ple and in 12 cases out of 12 we find the lensing galaxies in the optical and/or near infrared. Our success in finding lensing galaxies creates problems for the dark lens hypothesis. If it is to survive, ad hoc modifications seem to be necessary: only very massive galaxies (M> 9 1011M) can be dark, and the cut-off in mass must be sharp. Our finding of lens galaxies in all the JVAS/CLASS systems is complementary evidence which sup-ports the conclusion of Kochanek et al. (1997) that many of the wide-separation optically-selected pairs are physically distinct quasars rather than gravitational lens systems. Key words: galaxies: fundamental parameters – cosmology: dark matter – gravitational lensing 1