Galaxy Groups Associated with Gravitational Lenses and H_0 from   B1608+656
  Compact groups of galaxies recently have been discovered in association with several strong gravitational lens systems. These groups provide additional convergence to the lensing potential and thus affect the value of H_0 derived from the systems. Lens system time delays are now being measured with uncertainties of only a few percent or better. Additionally, vast improvements are being made in incorporating observational constraints such as Einstein ring structures and stellar velocity dispersions into the lens models. These advances are reducing the uncertainties on H_0 to levels at which the the effects of associated galaxy groups may contribute significantly to the overall error budget. We describe a dedicated multiwavelength program, using Keck, HST, and Chandra, to find such groups and measure their properties. We present, as a case study, results obtained from observations of the CLASS lens system B1608+656 and discuss the implications for the value of H_0 derived from this system. 
Chris Fassnacht (1), Lori Lubin (1), John McKean (1), Roy Gal (1),   Gordon Squires (2), Leon Koopmans (3), Tommaso Treu (4)...