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Published June 11, 2007 | Published + Accepted Version
Journal Article Open

High-resolution imaging of the anomalous flux ratio gravitational lens system CLASS B2045+265: dark or luminous satellites?


The existence of flux ratio anomalies between fold and cusp images in galaxy-scale strong-lens systems has led to an interpretation based on the presence of a high mass fraction of cold dark matter (CDM) substructures around galaxies, as predicted by numerical N-body simulations. These substructures can cause large perturbations of the image magnifications, leading to changes in the image flux ratios. The flux ratio anomaly is particularly evident in the radio-loud quadruple gravitational lens system CLASS B2045+265. In this paper, new high-resolution radio, optical and infrared imaging of B2045+265 is presented which sheds more light on this anomaly and its possible causes. First, deep Very Long Baseline Array observations show very compact images, possibly with a hint of a jet, but with no evidence for differential scattering or scatter broadening. Hence, the flux ratio anomaly is unlikely to be caused by refractive scattering in either the Milky Way or the lens galaxy. Secondly, optical and infrared observations with the Hubble Space Telescope and through adaptive optics imaging with the W. M. Keck Telescope, show a previously undiscovered object – interpreted as a (tidally disrupted) dwarf satellite based on its colours and slight extension – between the main lens galaxy and the three anomalous flux ratio images. Thirdly, colour variations in the early-type lens galaxy indicate recent star formation, possibly the result of secondary infall of gas-rich satellites. A population of such galaxies around the lens system could explain the previously discovered strong [O II] emission. However, spiral structure and/or normal star formation in the lens galaxy cannot be excluded. In light of these new data, we propose a lens model for the system, including the observed dwarf satellite, which reproduces all positional and flux ratio constraints, without the need for additional CDM substructure. Although the model is peculiar in that the dwarf galaxy must be highly flattened, the model is very similar to recently proposed mass models based on high-order multipole expansions.

Additional Information

© 2007 The Authors. Journal compilation © 2007 RAS. Accepted 2007 March 16. Received 2007 March 15; in original form 2006 October 23. We thank Richard Porcas and Peter Schneider for useful comments and discussions. We also thank Hendrik Hildebrandt for his advice on calculating photometric redshifts. The VLBA is operated by the National Radio Astronomy Observatory which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The results presented herein were based on observations collected with the NASA/ESA HST, obtained at STScI, which is operated by AURA, under NASA contract NAS5-26555. These observations are associated with programme numbers 6629 and 9744. 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. CEF acknowledges support from NSF's Research Experience for Undergraduates program, PHY-0243904. LVEK acknowledges partial support for this research from a VIDI grant (639.042.505) of the Netherlands Organization for Scientific Research (NWO). DT and BTS were supported by the Spitzer Space Telescope project. DT is also supported by LTSA grant NRA-00-01-LTSA-064. This work is supported by the European Community's Sixth Framework Marie Curie Research Training Network Programme, Contract No. MRTN-CT-2004-505183 'ANGLES'.

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Published - mnras0378-0109.pdf

Accepted Version - 0611215


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