The signature of substructure on gravitational lensing in the CDM cosmological model

¹ Institut für Astrophysik und Extraterrestrische Forschung, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: marusa@astro.uni-bonn.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ European Southern Observatory, Karl-Schwarzschild Str. 2, 85748 Garching bei München, Germany
⁴ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁵ Steward Observatory, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁶ Kapteyn Astronomical Institute, PO Box 800, 9700AV Groningen, The Netherlands
⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁸ Department of Physics and Astronomy, University of Victoria, BC V8P 1A1, Canada

Received: 12 June 2003
Accepted: 10 May 2004

Abstract

We present a study of the lens properties of quadruply imaged systems, lensed by numerically simulated galaxies. We investigate a simulated elliptical and disc galaxy drawn from high resolution simulations of galaxy formation in a concordance ΛCDM universe. The simulations include the effects of gas dynamics, star formation and feedback processes. Flux-ratio anomalies observed in strong gravitational lensing potentially provide an indicator for the presence of mass substructure in lens galaxies as predicted from CDM simulations. We particularly concentrate on the prediction that, for an ideal cusp caustic, the sum of the signed magnifications of the three highly magnified images should vanish when the source approaches the cusp. Strong violation of this cusp relation indicates the presence of substructure, regardless of the global, smooth mass model of the lens galaxy. We draw the following conclusions: (1) the level of substructure present in simulations produces violations of the cusp relation comparable to those observed; (2) higher-order catastrophes (e.g. swallowtails) can also cause changes of the order of 0.6 in the cusp relation as predicted by a smooth model; (3) the flux anomaly distribution depends on the image parity and flux and both the brightest minimum and saddle-point images are more affected by substructure than the fainter images. In addition, the brightest saddle point is demagnified w.r.t. the brightest minimum. Our results are fully numerical and properly include all mass scales, without making semi-analytic assumptions. They are ultimately limited by the mass resolution of single particles in the simulation determined by current computational limits, however show that our results are not affected by shot-noise due to the finite number of particles.