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Published May 1, 2006 | Published
Journal Article Open

Mass along the Line of Sight to the Gravitational Lens B1608+656: Galaxy Groups and Implications for H_0


We report the discovery of four groups of galaxies along the line of sight to the B1608+656 gravitational lens system. One group is at the redshift of the primary lensing galaxy (z = 0.631) and appears to have a low mass, with eight spectroscopically confirmed members and an estimated velocity dispersion of 150 ± 60 km s^(-1). The three other groups are in the foreground of the lens. These groups contain ~10 confirmed members each and are located at redshifts of 0.265, 0.426, and 0.52. Two of the three additional groups are centered roughly on the lens system, while the third is centered ~1' south of the lens. We investigate the effect of each of the four groups on the gravitational lensing potential of the B1608+656 system, with a particular focus on the implications for the value of H_0 derived from this system. We find that each group provides an external convergence of ~0.005-0.060, depending on the assumptions made in the calculation. For the B1608+656 system, the stellar velocity dispersion of the lensing galaxy has been measured, thus breaking the mass sheet degeneracy due to the group that is physically associated with the lens. The effect of the other groups along the line of sight can be folded into the overall uncertainties due to large-scale structure (LSS) along the line of sight. Because B1608+656 appears to lie along an overdense line of sight, the LSS will cause the measurement of H_0 to be biased high for this system. This effect could be 5% or greater.

Additional Information

© 2006 American Astronomical Society. Received 2005 October 25; accepted 2006 January 4. Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO-10158. We thank Leon Koopmans, Tommaso Treu, and Tony Tyson for useful discussions. We thank the anonymous referee for his or her comments. C. D. F. and J. P. M. acknowledge support under HST program GO-10158. Support for program GO-10158 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations would not have been possible without the expertise and dedication of the staffs of the Palomar and Keck observatories. We especially thank Paola Amico, Karl Dunscombe, Grant Hill, Jean Mueller, Ron Quick, Kevin Rykoski, Gabrelle Saurage, Chuck Sorenson, Skip Staples, Wayne Wack, Cindy Wilburn, and Greg Wirth. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawai'ian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This work is supported in part by the European Community's Sixth Framework Marie Curie Research Training Network Programme, contract MRTN-CT-2004-505183 "ANGLES." Facilities: HST (ACS, WFPC2), Keck:I (LRIS), Keck:II (ESI), PO:1.5m (CCD13)

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