Radio, optical and infrared observations of CLASS B0128+437
  We present new observations of the gravitational lens system CLASS B0128+437. HST observations detect a very faint, extended object in I-band with no 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, but 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 VLBI radio images at three frequencies, 2.3, 5 and 8.4GHz, made with the VLBA and the 100-m Effelsberg telescope. The lensed source consists of three well-defined sub-components embedded in a more extended jet. Due to the fact that the sub-components have different spectral indices it is possible to determine 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 scatter-broadening 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. 
A. D. Biggs (1 and 2), I. W. A. Browne (2), N. J. Jackson (2), T. York   (2), M. A. Norbury (2), J. P. McKean (2), P. M. Phi...