Observations of CLASS gravitational lenses and the unlensed parent population

This thesis presents observations of individual Cosmic Lens All-Sky Survey (CLASS) gravitational lenses and a study of the CLASS unlensed parent population. The selection of complete samples of flat-spectrum radio sources between 5 to 15 mJy (86 sources) and 25 to 50 mJy (45 sources) is presented. The 5 to 15 mJy sample was selected from a 4.85 GHz Pseudo-Survey and the 25 to 50 mJy sample was selected from CLASS. Together these two samples give a good statistical representation of the unlensed parent population. The number counts of flat-spectrum radio sources are shown to be consistent with the power-law, n(S[485] > 5 mJy) alpha 5[-2-06+/-0.01] The radio spectra of 26 sources from the 5 to 15 mJy and 44 sources from the 25 to 50 mJy samples were measured between 4.86 and 22.46 GHz with the VLA. Around 45 per cent of the 25 to 50 mJy sample was found to have strongly peaked spectra. Assuming a synchrotron self-absorbed emission mechanism is responsible for these spectra, the angular size of these radio sources is found to be < 6 mas. Attempts have been made to obtain redshifts for the complete samples from both optical spectroscopy and from optical and infra-red photometry. Redshift completeness is modest because many of the objects are very faint and do not have strong emission lines. It is estimated that the mean redshift of the 5 to 15 mJy and 25 to 50 mJy samples is z = 1.27 dz 0.24 (17 per cent completeness) and z = 1.15 +/- 0.23 (38 per cent completeness) respectively. The optical spectra of four CLASS gravitational lenses were taken with the W. M. Keck-I Telescope. The redshift of the background source in CLASS B0128+437 (z[s] = 3.124 +/- 0.004) and the lensing galaxy redshifts in CLASS B0445+123 (z[l,1] = 0.558 +/- 0.001), CLASS B0631+519 (z[l] = 0.090 +/- 0.001, z[l,2] = 0.620 +/- 0.001) and CLASS B0850+054 [z[l] = 0.587 +/- 0.001) were obtained. The lensing galaxies in CLASS B0445+123 and CLASS B0631+519 are early-type galaxies with Einstein Radii of 2.8 - 3.0h[-1] kpc. The deflector in CLASS B0850-I-054 is a late-type galaxy with an Einstein Radius of 1.6[-1] kpc. New optical and radio observations of the gravitational lens CLASS B2108-I-213 are presented. A deep MERLIN 5 GHz image shows the radio emission from one component to be extended while the other two are compact. The radio spectrum of the extended component also differs from that of the others. Therefore it is concluded that the radio emission from the lensing galaxy is being seen and not a third lensed image. Optical /-band imaging identifies two lensing galaxies within the Einstein Radius of the system. A simple mass model is also presented.