Global 8.4-GHz VLBI observations of JVAS B0218+357

In this paper we present new observations of the gravitational lens system JVAS B0218+357 made with a global VLBI network at a frequency of 8.4 GHz. Our maps have an rms noise of 30 μJy beam and with these we have been able to image much of the extended structure of the radio jet in both the A and B images at high resolution (∼1 mas). The main use of these maps will be to enable us to further constrain the lens model for the purposes of H0 determination. We are able to identify several sub-components common to both images with the expected parity reversal, including one which we identify as a counter-jet. We have not been successful in detecting either the core of the lensing galaxy or a third image. Using a model of the lensing galaxy we have back-projected both of the images to the source plane and find that they agree well. However, there are small, but significant, differences which we suggest may arise from multi-path scattering in the ISM of the lensing galaxy. We also find an exponent of the radial mass distribution of β ≈ 1.04, in agreement with lens modelling of published 15-GHz VLBI data. Polarisation maps of each image are presented which show that the distributions of polarisation across images A and B are different. We suggest that this results from Faraday rotation and associated depolarisation in the lensing galaxy.