2005/10/07 by Arthur B. Congdon, Charles R. Keeton · 1 citation
Physics and Astronomy · #Astrophysics #Astrophysics and Cosmic Phenomena #Classical mechanics #Computational physics #Dark matter #Galaxies: Formation, Evolution, Phenomena #Galaxy #Gravitation #Gravitational lens #Multipole expansion #Physics #Quantum mechanics #Radio Astronomy Observations and Technology #Redshift #Substructure #Theoretical physics #astro-ph
paper · pdf · doi:10.1111/j.1365-2966.2005.09699.x
published as Mon.Not.Roy.Astron.Soc.364:1459-1566,2005 · 10 pages, accepted in MNRAS
arxiv created 2005/10/07 · openalex publication_date 2005/11/14 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
It has been known for over a decade that many four-image gravitational lenses exhibit anomalous radio flux ratios. These anomalies can be explained by adding a clumpy cold dark matter (CDM) component to the background galactic potential of the lens. As an alternative, Evans & Witt (2003) recently suggested that smooth multipole perturbations provide a reasonable alternative to CDM substructure in some but not all cases. We generalize their method in two ways so as to determine whether multipole models can explain highly anomalous systems. We carry the multipole expansion to higher order, and also include external tidal shear as a free parameter. Fitting for the shear proves crucial to finding a physical (positive-definite density) model. For B1422+231, working to order kmax= 5 (and including shear) yields a model that is physical but implausible. Going to higher order (kmax≳ 9) reduces global departures from ellipticity, but at the cost of introducing small-scale wiggles in proximity to the bright images. These localized undulations are more pronounced in B2045+265, where kmax˜ 17 multipoles are required to smooth out large-scale deviations from elliptical symmetry. Such modes surely cannot be taken at face value; they must indicate that the models are trying to reproduce some other sort of structure. Our formalism naturally finds models that fit the data exactly, but we use B0712+472 to show that measurement uncertainties have little effect on our results. Finally, we consider the system B1933+503, where two sources are lensed by the same foreground galaxy. The additional constraints provided by the images of the second source render the multipole model unphysical. We conclude that external shear must be taken into account to obtain plausible models, and that a purely smooth angular structure for the lens galaxy does not provide a viable alternative to the prevailing CDM clump hypothesis.