Strong Gravitational Lensing

Strong lensing - multiple imaging of a background object by the gravity of a massive foreground object - is used to probe mass distributions in the Universe.

A smiling lens
In the centre of this image, taken with the NASA/ESA Hubble Space Telescope, are two faint galaxies that seem to be smiling. You can make out two orange eyes and a white button nose. In the case of this “happy face”, the two eyes are the galaxies SDSSCGB 8842.3 and SDSSCGB 8842.4 and the misleading smile lines are actually arcs caused by an effect known as strong gravitational lensing. Credit: NASA & ESA.

Our research focuses on systems in which our line of sight to a distant galaxy or quasar lies very close to a foreground galaxy. Under these circumstances the gravitational field of the foreground galaxy deflects the light from the background object, producing distorted, magnified, multiple images of the background object. The overall effect is known as strong gravitational lensing. These systems are useful because they provide magnified views of background objects, allowing us to study objects otherwise inaccessible to today's telescopes. They also give unique information about mass distributions in the lensing galaxy, as they are sensitive to all matter regardless of whether it emits light. Studies of lens systems are also important in cosmology.

Research activities

  • Studies of radio-quiet quasars - very faint radio sources

    Studying the nature of radio-quiet quasars using gravitational lensing.

    Quasars are galaxies in which most of the optical emission comes from a very bright region around the central black hole, rather than from stars in the galaxy.

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    Images of the radio-quiet quasar HS0810+2554.

    A minority of quasars emit bright radio jets from the very central regions areound the black hole, and are easily studied with radio telescopes. Most, however, emit only very faint radio emission, which is too faint to study easily. However, if an intervening galaxy lenses the background radio-quiet quasar, we can then study it. We are using lensing magnification to study the properties of radio emission from radio-quiet quasars and to understand its origin. We can analyse these images to produce a simple picture of the source (left hand panel) and measure its size. This would have been impossible with current telescopes in the absence of lensing.

  • Studies of central regions of galaxies

    In addition to the bright images of strongly lensed objects, we expect to see a very faint image which forms directly along the line of sight. Although faint, this image is very interesting as it carries information about the mass distribution of the lensing galaxy in the region very close to its central black hole. We have an e-MERLIN programme which is aimed at detecting these "central" images, which should now be accessible given the greatly increased sensitivity of this instrument.

  • New lens searches

    New instruments, both space-based such as Euclid, and ground-based such as the Large Synoptic Survey Telescope (LSST) and the Square Kilometre Array (SKA) will conduct surveys in the next 10 years which will be crucial in turning up tens of thousands of lens systems. These will allow major progress in studying mass distributions in galaxies, studying cosmology, and helping to understand galaxy evolution. We are participating in a number of these efforts.

  • Researchers

    Gravitational lensing in action. Credit: NASA, ESA & L. Calçada.

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