Radio , optical and infrared observations of CLASS B 0128 + 437

We present new observations of the gravitational lens system CLASS B0128+437 made in the optical, infrared and radio regimes. HubbleSpaceT elescope observations detect only a very faint, extended object in I-band with no obvious emission from the lensed images visible; no detection at all is made in V -band. The lens system is detected with much higher signal to noise with UKIRT in K-band and, although resolved, the resolution is not sufficient to allow the lensed images and the lens galaxy to be separated. A careful astrometric calibration, however, suggests that the peak of the infrared emission corresponds to the two merging images A and B and therefore that the lensed images dominate at infrared wavelengths. The new radio data consist of high resolution VLBI radio images at three frequencies, 2.3, 5 and 8.4 GHz, made with the VLBA and the 100-m Effelsberg telescope. These reveal that the lensed source consists of three well-defined sub-components that are embedded in a more extended jet. Due to the fact that the sub-components have different spectral indices it is possible to determine, unambiguously, which part of each image corresponds to the same source sub-component. Our main finding is that one of the images, B, looks very different to the others, there being no obvious division into separate sub-components and the image being apparently both broader and smoother. This is a consequence we believe of scatterbroadening in the ISM of the lensing galaxy. The large number of multiply-imaged source sub-components also provide an abundance of modelling constraints and we have attempted to fit an SIE+external shear model to the data, as well as utilising the novel method of Evans & Witt. It proves difficult in both cases, however, to obtain a satisfactory fit which strongly suggests the presence of sub-structure in the mass distribution of the lensing galaxy, perhaps of the kind that is predicted by CDM theories of structure formation.