Lensing of extended sources by dark galactic halos

Abstract

Two aspects of gravitational lensing are mainly addressed in this paper: (i) the role of dark matter as a lensing agent and (ii) imaging of extended background sources. An attempt is made to reproduce the observed image configurations of lens systems 2016 + 112, 0023 + 171, 2345 + 007, and 1635 + 267 by invoking the assistance of dark halos around the lensing galaxies. The models that can reproduce the observed features of these systems reasonably well consist of normal galaxies accompanied by galactic halos of size 75-150 kpc and mass 1-2 × 10¹³_⊙. It is demonstrated that from the observational standpoint the image features resulting from the lensing of extended sources are not reproduced by the models based only on point sources. Speculative arguments are presented about the nature of an unobserved lensing agent in the case of 2345 + 007 and 1635 + 267. Should a substantial amount of matter exist in the universe in the form of nonluminous clumps of size ~ 50kpc and mass ~10¹²_⊙, obeying the Schuster law of density distribution, then the lensing action of individual clumps can account for some of the observed image configurations. A promising method to detect such a lens in the case of 2345 + 007 would be to undertake radio observations of the 21 cm line emission at the ~10 mJy level in the frequency range of 500-900 MHz.