Proceedings of the International Astronomical Union

Impact of Gravitational Lensing on Cosmology
Contributed Papers

Galaxy Groups Associated with Gravitational Lenses and $H_0$ from B1608+656

Chris Fassnacht a1, Lori Lubin a1, John McKean a1, Roy Gal a1, Gordon Squires a2, Leon Koopmans a3, Tommaso Treu a4, Roger Blandford a5 and David Rusin a6
a1 Department of Physics, UC Davis, 1 Shields Ave., Davis, CA 95616, USA email:,,,
a2 Spitzer Science Center, California Institute of Technology, Mail Code 220-6, 1200 East California Boulevard, Pasadena, CA 91125, USA email:
a3 Kapteyn Institute, PO Box 800, 9700AV Groningen, The Netherlands email:
a4 Department of Physics and Astronomy, UCLA, Box 951547, Knudsen Hall, Los Angeles, CA 90095, USA email:
a5 KIPAC, Stanford University, 2575 Sand Hill Road, Menlo Park, CA 94025, USA email:
a6 Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA email:

Article author query
fassnacht c   [Google Scholar] 
lubin l   [Google Scholar] 
mckean j   [Google Scholar] 
gal r   [Google Scholar] 
squires g   [Google Scholar] 
koopmans l   [Google Scholar] 
treu t   [Google Scholar] 
blandford r   [Google Scholar] 
rusin d   [Google Scholar] 
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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.


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