Studies of two CLASS gravitational lenses

This work describes observations of two strong gravitational lenses found in the JVAS/CLASS radio surveys, JVAS B0218+357 and CLASS B0631+519, together with modelling of the mass distribution in B0631+519 and in a third object, CLASS B0128+437. CLASS B0631+519 was observed with the VLBA and MERLIN at 1.7 GHz. The MERLIN data show that the lensed source consists of a doubly-imaged flat- spectrum region located between two steep-spectrum lobes, one doubly-imaged and the other quadruply-imaged. The VLBA observations were compared with data taken at the same frequency three years earlier. No compelling evidence for variability was found above the 2sigma level. The VLBA data also allowed the flat- spectrum core to be identified in the sub-components making up the flat-spectrum region. NICMOS and ACS observations of B0631-f519 were used to establish the lens galaxy position, and to estimate the redshift of the lens. The ACS data confirm that there are two galaxies present in this system, although one appears to have little effect on the lensed source. The near-infrared NICMOS data show an Einstein ring, which will be of use in constraining more complex mass models. The gravitational lens JVAS B0218-I-357 is nearly a perfect target for attempts to measure Hubble's constant, H0. Measuring the lens galaxy's location relative to the lensed images is a prerequisite for this objective. Data taken with the ACS indicate a galaxy position 60 mas west of the brightest image of the source. The resulting value of H0 is 68+/-6 km s-1 Mpc-1. However, the position is sensitive to the spiral arms of the lens. When they are masked out, the measured galaxy position shifts by 16 mas to the south-west, giving H0 = 56[+12-15] km s-1 Mpc-1. The optical position measurement can also be combined with constraints from VLBI observations of the lens; without masking of the spiral arms, the combined constraints produce H0 = 70+/-5 km s-1 Mpc-1, or 61+/-7 km s-1 Mpc-1 with masking.