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Posted: Sep 06, 2013

The quasar and its Fata Morgana

(Nanowerk News) Bonn astronomers discover how the image of a distant quasar splits into multiple images by the effects of a cloud of ionized gas in our own Milky Way Galaxy. Such events were predicted as early as in the 1970s, but the first evidence for one now has come from observations performed with the telescope array VLBA and analysed in the Max Planck Institute for Radio Astronomy ("VLBA observations of a rare multiple quasar imaging event caused by refraction in the interstellar medium").
how radio waves from the distant quasar jet are bent by a gas cloud in our own Galaxy
Artist's diagram of the refraction event (not drawn to scale), showing how radio waves from the distant quasar jet are bent by a gas cloud in our own Galaxy, creating multiple images seen with the Very Long Baseline Array. ( Bill Saxton, NRAO/AUI/NSF)
The scientists observed the quasar 2023+335, nearly 3 billion light-years from Earth, as part of a long-term study of ongoing changes in some three hundred quasars. When they examined a series of images of 2023+335, they noted dramatic differences. The differences, they said, are caused by the radio waves from the quasar being bent as they pass through the Milky Way gas cloud, which moved through our line of sight to the quasar. "So as we would see a spot of light get broader or even multiple behind a frosted glass, we see this quasar 'dancing' behind a gas cloud in our own Galaxy", so Anton Zensus, Director at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, and member of the international team who has discovered this effect. "This is similar to the fata morgana to be seen in the desert or to the Sun dogs caused by iced clouds with the image of our own star", adds Zensus.
"This event, obviously rare, gives us a new way to learn some of the properties of the turbulent gas that makes up a significant part of our Galaxy," said Alexander Pushkarev from the MPIfR in Bonn, Germany, and the Crimean Astrophysical Observatory, Ukraine, and leader of the international team.
New insights into turbulent galactic gas clouds become tangible
The scientists added 2023+335 to their list of observing targets in 2008. Their targets, in the framework of the MOJAVE project, are quasars and other galaxies with supermassive black holes at their cores. The gravitational energy of the black holes powers "jets" of material propelled to nearly the speed of light. The quasar 2023+335 initially showed a typical structure for such an object, with a bright core and a jet. In 2009, however, the object's appearance changed significantly, showing what looked like a line of bright, new radio-emitting spots.
"We've never seen this type of behaviour before, either among the hundreds of quasars in our own observing program or among those observed in other studies," adds Eduardo Ros from the MPIfR, also a team member in the discovery.
Gas clouds could also refract the light of other quasars
The multiple-imaging event came as other telescopes detected variations in the radio brightness of the quasar, caused, the astronomers said, by scattering of the waves.
Galactic plan
Artist's impression of a face-on view of the Galactic plane. The dashed line shows the direction towards quasar 2023+335, passing through the nearby Cygnus region in the Local arm of our Galaxy, the Milky Way. ( R. Hurt, NASA/JPL-CalTech/SSC; taken from Pushkarev et al.)
The scientists' analysis indicates that the quasar's radio waves were bent by a turbulent cloud of charged gas nearly 5,000 light-years from Earth in the direction of the constellation Cygnus. The cloud's size is roughly comparable to the distance between the Sun and Mercury, and the cloud is moving through space at about 56 kilometres per second (or 200.000 km/h, comparable to the speed of Helios 2, the fastest spacecraft constructed ever).
"Monitoring of 2023+335 over time may yield more such events, so we can learn additional details both about the process by which the waves are scattered and about the gas that does the scattering. Other quasars that are seen through similar regions of the Milky Way also may show this behaviour", concludes Pushkarev.
The monitoring program that yielded this discovery is called MOJAVE (Monitoring Of Jets in Active galactic nuclei with VLBA Experiments), run by an international team of scientists led by Matt Lister from Purdue University. The researchers recently published their results in the journal Astronomy and Astrophysics.
Source: Max Planck Institute for Radio Astronomy
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