Strong Lensing

Introduction

When the light from a distant object is bent to such a great extent that we, the observer, can see more than one image of the same source then we are in the strong lensing regime. Technically this can be defines as the region where the 2D mass distribution is greater than the critical density (For more details see General lensing). This page is designed to give a brief overview of strong lensing systems and to show some well known results. These will all use the thin lens approximation where the observer, lens and source are on fixed planes.

In strong lensing we can use the information coming from the multiple images to study the lens, the source and some general cosmology to do with the geometry of the Universe.

Examples of strong lenses

Astronomical observational are full of examples of multiple images systems. In fact due to the extent of the lensing signal, strong lenisng systems produce some of the most striking images in astronomy. Most of us working in this field usually like to illustrate the lensing effect but showing images of galaxy clusters. Clusters are the most massive (collapsed) objects in the Universe. They typically have a mass of 1015 greater than the mass of our sun. This makes them over 1000 times more massive than our own galaxy, the Milky Way. Figure SL1 shows and image of one such galaxy cluster (Abell 2218) that was taken using the Hubble Space Telescope. In this image we see a large number of giant arcs that seem to form a circular feature that surounds the brightest galaxy in the image. These arcs are the images of galaxies that are behind the cluster. In the absence of lensing the images from these galaxies would have been fairly round, however the mass of the cluster has bent and distorted the light from these galaxies to such an extent that we see them as long stretched arcs

Five Easy Steps

1. Exercise 1
2. Exercise 2
3. Exercise 3
4. Exercise 4
5. Exercise 5

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