Astronomers Dissect a Supermassive Black Hole with Natural Magnifying Glasses

ESO PR Photo 47a/08
The Einstein Cross
Credit: ESO/F. Courbin et al.

About this image: The Einstein Cross and the galaxy that causes this 'cosmic mirage', as seen with the FORS instrument on ESO's Very Large Telescope. This cross-shaped configuration consists of four images of a single very distant source. The multiple images are a result of gravitational lensing by a foreground galaxy, an effect that was predicted by Albert Einstein as a consequence of his theory of general relativity. The light source in the Einstein Cross is a quasar approximately ten billion light-years away, whereas the foreground lensing galaxy is ten times closer. The light from the quasar is bent in its path and magnified by the gravitational field of the lensing galaxy.

Sunday, December 14, 2008



ESO PR Photo 47b/08
The Einstein Cross
Credit: ESO/F. Courbin et al.

About this image: Close-up of the Einstein Cross, as observed with the SINFONI instrument on ESO's Very Large Telescope. SINFONI makes use of the adaptive optics technique and so, allows astronomers to overcome the blurring effect of the atmosphere, thereby providing very sharp images. The central blob is the nucleus of the lensing galaxy, surrounded by the four mirage images of the distant quasar.



Macro and microlensing

This animation shows the principle of macro- and microlensing. In "macrolensing", a galaxy plays the role of a cosmic magnifying glass or a natural telescope, an effect that was predicted by Albert Einstein as a consequence of his theory of general relativity. The light from a distant quasar is bent in its path and magnified by the gravitational field of the lensing galaxy. This proves very useful in astronomy as it allows us to observe distant objects that would otherwise be too faint to explore using currently available telescopes. In addition to macrolensing by the galaxy, stars in the lensing galaxy act as secondary lenses to produce an additional magnification. This secondary magnification is based on the same principle as macrolensing, but on a smaller scale, and since stars are much smaller than galaxies, is known as "microlensing". As the stars are moving in the lensing galaxy, the microlensing magnification also changes with time. From Earth, the brightness of the quasar images (four in the case of the Einstein Cross) flickers around a mean value, due to microlensing.
Credit: ESO

Combining a double natural "magnifying glass" with the power of ESO's Very Large Telescope, astronomers have scrutinised the inner parts of the disc around a supermassive black hole 10 billion light-years away. They were able to study the disc with a level of detail a thousand times better than that of the best telescopes in the world, providing the first observational confirmation of the prevalent theoretical models of such discs.

The team of astronomers from Europe and the US studied the "Einstein Cross", a famous cosmic mirage. This cross-shaped configuration consists of four images of a single very distant source. The multiple images are a result of gravitational lensing by a foreground galaxy, an effect that was predicted by Albert Einstein as a consequence of his theory of general relativity. The light source in the Einstein Cross is a quasar approximately ten billion light-years away, whereas the foreground lensing galaxy is ten times closer. The light from the quasar is bent in its path and magnified by the gravitational field of the lensing galaxy.

This magnification effect, known as "macrolensing", in which a galaxy plays the role of a cosmic magnifying glass or a natural telescope, proves very useful in astronomy as it allows us to observe distant objects that would otherwise be too faint to explore using currently available telescopes. "The combination of this natural magnification with the use of a big telescope provides us with the sharpest details ever obtained," explains Frédéric Courbin, leader of the programme studying the Einstein Cross with ESO's Very Large Telescope.

In addition to macrolensing by the galaxy, stars in the lensing galaxy act as secondary lenses to produce an additional magnification. This secondary magnification is based on the same principle as macrolensing, but on a smaller scale, and since stars are much smaller than galaxies, is known as "microlensing". As the stars are moving in the lensing galaxy, the microlensing magnification also changes with time. From Earth, the brightness of the quasar images (four in the case of the Einstein Cross) flickers around a mean value, due to microlensing. The size of the area magnified by the moving stars is a few light-days, i.e., comparable in size to the quasar accretion disc.

The microlensing affects various emission regions of the disc in different ways, with smaller regions being more magnified. Because differently sized regions have different colours (or temperatures), the net effect of the microlensing is to produce colour variations in the quasar images, in addition to the brightness variations. By observing these variations in detail for several years, astronomers can measure how matter and energy are distributed about the supermassive black hole that lurks inside the quasar. Astronomers observed the Einstein Cross three times a month over a period of three years using ESO's Very Large Telescope (VLT), monitoring all the brightness and colour changes of the four images.

"Thanks to this unique dataset, we could show that the most energetic radiation is emitted in the central light-day away from the supermassive black hole and, more importantly, that the energy decreases with distance to the black hole almost exactly in the way predicted by theory," says Alexander Eigenbrod, who completed the analysis of the data.

The use of the macro- and microlensing, coupled with the giant eye of the VLT, enabled astronomers to probe regions on scales as small as a millionth of an arcsecond. This corresponds to the size of a one euro coin seen at a distance of five million kilometres, i.e., about 13 times the distance to the Moon! "This is 1000 times better than can be achieved using normal techniques with any existing telescope," adds Courbin.

Measuring the way the temperature is distributed around the central black hole is a unique achievement. Various theories exist for the formation and fuelling of quasars, each of which predicts a different profile. So far, no direct and model-independent observation has allowed scientists to validate or invalidate any of these existing theories, particularly for the central regions of the quasar. "This is the first accurate and direct measurement of the size of a quasar accretion disc with wavelength (colour), independent of any model," concludes team member Georges Meylan.

Record-breaking cosmic mirage

photo: This picture, taken by the Subaru Telescope on the summit of Mauna Kea in Hawaii, shows four images of the same quasar (the four white dots in the center). The quasar is almost 10 billion light-years from us and its light has been split into four by the gravitational influence of a foreground cluster of galaxies 6.2 billion light-years from us. Some of the galaxies of the cluster appear as yellow dots in the image. Sloan Digital Sky Survey


January 1, 2004

A widely lensed quasar reveals the presence of dark matter.Having something block your view when taking a photo can be a real pain — except perhaps when studying cosmic mirages and dark matter.

Astronomers have discovered a gravitationally lensed quasar located more than 10 billion light-years away that is shedding new light on dark matter in the universe. Parked behind a massive cluster of galaxies, the quasar's feeble light has been bent and split into four distorted images that have the largest angular separation ever found. According to the discovery team, this wide-angle effect is evidence that invisible cold dark matter dominates the foreground cluster and is responsible for the record-breaking quadruple mirage.

Since 1979, more than 80 gravitationally lensed quasars have been cataloged, however none were found to have separations of more than 7 arcseconds. Despite theoretical models that predicted larger splitting of quasar images, numerous searches had come up empty — until now.

Mining the colossal database of over 30,000 quasars from the Sloan Digital Sky Survey (SDSS), an international team of astronomers led by Naoisha Inada and Masamune Oguri from the University of Tokyo pinpointed SDSS J1004+4112 in the constellation Leo Minor.Using the Subaru Telescope on the summit of Mauna Kea, Hawaii, the group managed to identify four individually split quasar images separated by 14.62 arcseconds — more than twice as large as the previous record-holding lensed quasar.



photo: This 2.5-meter telescope is the main workhorse of the Sloan Digital Sky Survey. Its box-shaped structure protects it against the wind. SDSS Collaboration.

"Additional observations obtained at the Subaru 8.2-meter Telescope and Keck Telescope confirmed that this system is indeed a gravitational lens," explains lead author Inada. "Quasars split this much by gravitational lensing are predicted to be very rare, and thus can only be discovered in very large surveys like the SDSS."

First predicted by Einstein more than six decades ago, gravitational lensing occurs when the gravity from a massive foreground object bends and amplifies the light from a more distant object, as seen from Earth. Astronomers have been using this giant magnifying-lens effect to bring into view quasars and galaxies that otherwise would be too faint to detect. Some lensed quasars produce multiple images including, in rare cases (if the alignment is perfect), rings around the lensing galaxies.In the December 18 edition of Nature, the team argues that the quadruple lensing effect at J1004+4112 is caused by the gravitational influence of a cluster of galaxies about 6.2 billion light-years away. They believe that because the visible mass of this cluster cannot account for the observed 14.62-arcsecond separation, high concentrations of intervening material in the form of unseen cold dark matter must be causing this unprecedented wide splitting.



photo: In a gravitational lens, a foreground galaxy (here shown as a red spiral) causes light (dashed lines) from a background quasar to bend. As a result, an observer may see multiple quasars instead of just one. In most cases the quasar images are only offset by an arcsecond, but occasionally, as is shown here, the offset is much larger. Astronomy.com: Pamela L. Gay

Oguri added: "Discovering one such wide gravitational lens out of over 30,000 SDSS quasars surveyed to date is perfectly consistent with theoretical expectations of models in which the universe is dominated by cold dark matter. This offers additional strong evidence for such models."

The authors expect many more such wide-angle lensed quasars will be encountered and that they will become powerful tools in the study of the distribution of dark matter in the universe. "The gravitational lens we have discovered will provide an ideal laboratory to explore the relation between visible objects and invisible dark matter in the universe," adds Oguri.