Strong Lensing by Galaxies

The remainder of this book is concerned with applications of gravitational lensing of cosmological relevance. We proceed by considering ever larger mass and distance scales, starting with galaxy-scale strong lensing in the present chapter. The background sources in this context are point-like quasars or extended galaxies. As we will see in Sect. 6.1, the axisymmetric models of Chap. 2 can be readily adapted to the more general setting of Chap. 4. Only such non-axisymmetric mass distributions can be expected to describe observed lens systems. We noted in Chap. 2 that actual galaxies have finite density everywhere and offered the nonsingular isothermal sphere as an example. We extend that discussion to more general lenses in Sect. 6.2. Even all of these refinements are insufficient for understanding the full variety of observed galaxy lenses. For example, we must account for extended sources (Sect. 6.3) and for lenses that cannot be described by smooth density profiles (Sect. 6.4). Using the properties of observed images to infer the mass distribution of the lens is the subject of Sect. 6.5. Applications of the models and methods described in this chapter are outlined in Sect. 6.6.