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© 2012 EPFL

21.03.12 - To determine the mass of a galaxy, you need another galaxy. Using NASA and ESA’s Hubble Space Telescope, EPFL scientists have identified several rare cases of galaxies that contain supermassive black holes acting as gravitational lenses. The discovery will lead to a better understanding of how black holes are formed.

Two years ago, scientists in EPFL’s Laboratory of Astrophysics (LASTRO) discovered a quasar, a galaxy with a black hole at its center, that functioned as a gravitational lens, a kind of cosmic magnifying glass. The case was special for two reasons: it showed both the presence of the supermassive black hole as well as another galaxy in the background, very distant and in almost perfect alignment. The light coming from this distant galaxy, strongly bent by its passage near the black hole, made it possible to measure the mass of a quasar for the first time.

Since then, using NASA and ESA’s Hubble Space Telescope the scientists have discovered several other examples of these rare specimens, selected from among a sample of 20,000 quasars. Their results are published March 21, 2012 in the scientific journal Astronomy & Astrophysics.

This magnification phenomenon, called gravitational lensing, is caused by massive objects in the universe such as stars, galaxies, and planets. When the light from a very distant galaxy passes near one of these objects on its voyage to Earth, it is bent by the gravitational pull of the object. The image of the galaxy therefore appears severely distorted when it reaches us. There are either multiple images of the galaxy, or, if it’s in almost perfect alignment with the massive object, the image appears in the shape of a circular arc, known as an Einstein Ring. The size of the ring allows us to determine the mass of the object situated in the middle, which is acting like a lens. In the cases discovered by the EPFL scientists, the object in the foreground is a quasar, or a galaxy with a supermassive black hole at its center. It would be impossible to “weigh” the quasar without this gravitational lensing effect.

Better images

“We found much better examples than our first discovery, with clearer and more detailed images,” explains LASTRO physicist Frédéric Courbin. A tool like this is important, he says, because it allows us to calculate the total amount of matter, visible or invisible, contained in the quasar.

“For the first time, we have a reliable method for measuring the mass of these objects, which are too luminous to be observed with traditional techniques,” Courbin explains. “We can thus better understand why some galaxies have a black hole and others don’t, what their incredible energy is made up of, how the matter is distributed and how it evolves. The gravitational lensing effect has already taught us much about the distribution of matter in galaxy clusters and galaxies themselves. Here the effect is produced by very special objects, whose mass has been impossible to measure up to this point.”

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