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

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Letter to the Editor

Lensing galaxies: light or dark?

N. Jackson 1, P. Helbig 1, I. Browne 1, C.D. Fassnacht 2, L. Koopmans 3, D. Marlow 1 and P.N. Wilkinson 1

1 University of Manchester, NRAL, Jodrell Bank, Macclesfield SK11 9DL, UK
2 California Institute of Technology, Mail Code 105-24, Pasadena, CA 91125, USA
3 Kapteyn Astronomical Institute, Postbus 800, 9700 AA, Groningen, The Netherlands

Received 22 January 1998 / Accepted 24 February 1998

Abstract

In a recent paper, Hawkins (1997) argues on the basis 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 result 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 sample 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 ([FORMULA]) can be dark, and the cutoff in mass must be sharp. Our finding of lens galaxies in all the JVAS/CLASS systems is complementary evidence which supports 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

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© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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