Astrophysics Group

Cavendish Laboratory


Mullard Radio Astronomy Observatory
The Arcminute Microkelvin Imager (AMI)

The most recent telescope built at MRAO is the Arcminute Microkelvin Imager (AMI). This is a cosmic microwave background (CMB) telelescope with a difference, in that it is designed to survey the sky looking for galaxy clusters by means of the ‘shadows’ they cast against the CMB. This shadowing, the so-called SZ effect, is a remarkable phenomenon because it is independent of redshift, allowing AMI to find clusters which are so far away that their own emission would be too faint to detect – as expained more fully below.

As we look out to greater distances (higher redshifts) we are going back earlier into the Universe. As we try to find objects at earlier times, to see how populations of objects have changed with time, it gets progressively harder because the objects appear fainter and fainter the greater their redshift or distance from us. To circumvent this limitation, AMI adopts a different observing strategy. Instead of looking for clusters of galaxies by trying to detect the radiation they emit themselves, AMI uses the CMB as a backdrop of radiation against which clusters of galaxies (in the foreground) can be observed by means of the ‘shadows’ they cast on the CMB. In particular, AMI exploits the remarkable fact that this shadowing (the SZ effect) is independent of redshift, which means that it will be capable of detecting galaxy clusters as far away/early in time as they exist – even at redshifts where they would be too faint to detect via their own emission.

As well as early galaxy clusters, AMI may find other signatures of structure formation that occurred in what are currently the ‘Dark Ages’ – the epoch earlier than that of the most luminous distant quasars (at redshift around 6), whose own light we can just detect.

AMI actually comprises two interferometer arrays operating over a frequency range 13.5-18 GHz.

The Small Array consists of ten 3.7-m dishes:

The AMI Small Array

The Large Array consists of the eight 13-m dishes of the Ryle Telescope, in a new configuration:

The AMI Large Array

The different dish sizes allow a great range of baselines, so that AMI is sensitive to structures on a large range of angular scales.

For a more technical description of AMI and the underlying science, please visit the official AMI website.

For information on other MRAO telescopes, follow the links on the left.