A galaxy magnified by a quasar

The EPFL’s Laboratory of Astrophysics has, for the first time ever, observed a quasar that is located between the earth and a more distant galaxy. Acting as a gravitational lens, it makes new kinds of observations possible.

The galaxy is 7.5 billion light-years away from Earth, yet it shines brighter than it should. This bizarre physics phenomenon is caused by a quasar—a very distant and energetic galaxy that is situated between our planet and another galaxy—playing the role of a gravitational lens. The phenomenon has been observed for the first time by the EPFL’s Laboratory of Astrophysics in cooperation with Caltech, and published in the journal Astronomy & Astrophysics.

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.

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 world first in astrophysics

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.

A sample of some 23,000 of these quasars in the northern hemisphere was selected by the Laboratory of Astrophysics team. In the end, only four of them seemed to act as a gravitational lens.One of these was studied using the Keck telescope (Caltech) on Mauna Kea peak in Hawaii. These images will be supplemented in the coming months with very high-quality photographs from the Hubble Space Telescope, which will reveal more about the nature of this particular quasar.

Author: Sarah Perrin /Lionel Pousaz / trans. Michael David Mitchell

Source: EPFL