Adaptive Optics Observations of B0128+437: A Low-Mass, High-Redshift
  Gravitational Lens
  We use high-resolution adaptive optics (AO) imaging on the Keck II telescope
to study the gravitational lens B0128+437 in unprecedented detail, allowing us
to resolve individual lensed quasar components and, for the first time, detect
and measure properties of the lensing galaxy. B0128+437 is a small separation
lens with known flux-ratio and astrometric anomalies. We discuss possible
causes for these anomalies, including the presence of substructure in the
lensing galaxy, propagation effects due to dust and a turbulent interstellar
medium, and gravitational microlensing. This work on B0128 demonstrates that AO
will be an essential tool for studying the many new small-separation lenses
expected from future surveys.
