The Gravitational Lens G2237 + 0305

The European Space Agency's Faint Object Camera on board NASA's Hubble Space Telescope has provided astronomers with the most detailed image ever taken of the gravitational lens G2237 + 0305 – sometimes referred to as the "Einstein Cross." The photograph shows four images of a very distant quasar which has been multiple-imaged by a relatively nearby galaxy acting as a gravitational lens. The angular separation between the upper and lower images is 1.6 arcseconds.

The quasar seen here is at a distance of approximately 8 billion light-years, whereas the galaxy at a distance of 400 million light years is 20 times closer. The light from the quasar is bent in its path by the gravitational field of the galaxy. This bending has produced the four bright outer images seen in the photograph. The bright central region of the galaxy is seen as the diffuse central object.

Gravitational lensing occurs when the light from a distant source passes through or close to a massive foreground object. Depending on the detailed alignment of the foreground and background objects with the line of sight to Earth, several images of the background object may be seen. In fact, astronomers expect that a faint fifth image of the quasar should be present near the center of the galaxy in G2237 + 0305. Careful image processing will be needed to determine if the fifth image is actually seen in this FOC exposure.

Gravitational lenses, such as G2237 + 0305, are useful probes of many types of phenomena that occur in the cosmos. For example, it is possible to "weigh" the foreground galaxy by measuring the relative positions and the brightnesses of the different images of the quasar. This should be possible to do more accurately given the resolution of images obtained with the Faint Object Camera. Also, gravitational lenses in general offer the possibility of determining the elusive "Hubble Constant" – a fundamental measure of the size and age of the universe – by measuring the time delays in changes of the brightness of the lensed images.

Detailed analysis of this fascinating Faint Object Camera image and others to be observed later with the Hubble Space Telescope will provide a wealth of information on the details of lensing galaxies, as well as on the process of gravitational lensing itself.