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 very long baseline interferometry (VLBI) network at a frequency of 8.4 GHz. Our maps have an rms noise of 30 muJy beam(-1) 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 (similar to1 mas). The main use of these maps will be to enable us to further constrain the lens model for the purposes of H-o determination. We are able to identify several subcomponents 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 interstellar medium (ISM) of the lensing galaxy. We also find an exponent of the radial mass distribution of beta approximate to 1.04, in agreement with lens modelling of published 15-GHz VLBI data. Polarization maps of each image are presented which show that the distributions of polarization across images A and B are different. We suggest that this results from Faraday rotation and associated depolarization in the lensing, galaxy.