How does an object such as a galaxy split the image from a more distant galaxy into two images?

Question

I was reading a recent discussion on gravitational lensing, in which it described the bending of light from a distant galaxy as it passed a nearer galaxy to produce two images of the same distant galaxy.

How does that happen? If the distant object is directly behind the lensing object, the light would be bent around it "on all sides"; there shouldn't be two images of the distant object, but a sort of distorted "halo" surrounding the lensing object, correct?

And if the distant object is not directly behind the lensing object, its image would simply be displaced, not divided, correct?

So how does a gravitationally lensing object produce two distinct images of something more distant?

Answer 1

If the lens were a point source then this is more or less correct. You would have an Einstein ring image for perfect alignment, and as the alignment worsened the ring would rapidly collect into a displaced image of the source with minimal distortion.

However, the lens is very much not a point source. It is a galaxy or a cluster of galaxies. The question of which ways light rays are scattered becomes much more complicated. The analogy is often drawn with the pattern of sunlight at the bottom of a pool of not-still water, and indeed the analysis of caustics is qualitatively the same.