Quasar acting as a cosmic lens

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Light from a distant source, bent around a massive object. The white arrows show

Light from a distant source, bent around a massive object. The white arrows show the path of the light from the true position of the lightsource. The orange arrows show the apparent position of the lightsource. (NASA)

Astrophysicists have for the first time observed a quasar that is located between the earth and a more distant galaxy and acts as a gravitational lens. This phenomenon illustrates Albert Einstein’s theory of general relativity, and will make entirely new kinds of observations possible.

A quasar acting as a gravitational lens has now been observed for the first time. This discovery, made by the EPFL’s Laboratory of Astrophysics represents an advance in the field, since it will allow scientists to weigh and measure a galaxy that contains a quasar.

Gravitational lenses everywhere

Gravitational lenses are common throughout the universe. They are caused by massive objects such as stars or galaxies that bend rays of light passing nearby. If these objects are between the earth and a more distant light source, the light will therefore be brighter and easier to observe, but also very distorted. If the alignment of the various stellar bodies is almost perfect, the image of the source will be multiplied.

Interesting astrophysical tool

The lens phenomenon is not only an interesting result of Einstein’s theory of general relativity; it has also been a valuable astrophysical tool with important applications in the search for extrasolar planets and the study of stars, galaxies, clusters of galaxies and quasars. For example, the nature of the distortion, the number of images of the most distant objects and their position in the sky provide essential information about the distribution of matter in the lens galaxy and allow a measurement of its total matter, including dark matter, to be made. A quasar is the heart of a galaxy, consisting of a supermassive black hole. The small fraction of the galaxy’s mass that is close enough to be swallowed up by the black hole emits light before disappearing forever, giving rise to this extremely bright and transient phenomenon.

Possibility to “weigh” a galaxy

To date, about a hundred of these quasars emitting light that is concentrated by a lens galaxy located between them and the earth have been discovered. However, this is the first time that the opposite case has been observed, where the quasar is in the foreground and the galaxy behind it. The interest of this discovery lies in the fact that it provides an unprecedented opportunity to “weigh” a galaxy containing a quasar.

C. Wirth, Editor

 
 
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