More realistic lensing

In the previous section we assumed that both lens and source where just point masses. When the lens is extended, you can get multiple images (3,5,7,...), depending on the impact parameter and the mass distribution of the lens. If the source is extended as well, then different parts of the source may be deflected and amplified in various ways, and so the image may be highly deformed. We are now in a position to look for applications. To wet your appetite, let me show you two beautiful examples.

Figure 12.3 shows an Einstein ring. The lens is probably an elliptical galaxy, and the source a more distant spiral. Because of the near perfect alignment, the spiral has been imaged into a ring.

Figure 12.4 shows how a nearby spiral galaxy has lensed a very distant background quasar and produced multiple images of it, in this case five.

Figure 12.3: Hubble Space Telescope image of an Einstein ring, where a fortuitous alignment has caused an intervening elliptical galaxy (the central spot) to lens a background spiral galaxy into a ring.

Figure 12.4: Hubble Space Telescope image of the `Einstein cross', where a distant quasar has been lensed into multiple images by an intervening spiral galaxy.