Colors of Quasars Reveal a Dusty Universe

Spiral galaxies seen edge-on often show dark lanes of interstellar dust blocking light from the galaxy's stars, as in this image of the galaxy NGC 4565 from the Sloan Digital Sky Survey (SDSS-II). The dust is formed in the outer regions of dying stars, and it drifts off to mix with interstellar gas.
The new analysis of quasar colors shows that galaxies also expel dust to distances of several hundred thousand light years, ten times farther than the visible edge of the galaxy seen in this image. The thin haze of intergalactic dust dims and reddens the light from background quasars.

Credit: The Sloan Digital Sky Survey
Friday, February 27, 2009

The vast expanses of intergalactic space appear to be filled with a haze of tiny, smoke-like "dust" particles that dim the light from distant objects and subtly change their colors, according to a team of astronomers from the Sloan Digital Sky Survey (SDSS-II).

"Galaxies contain lots of dust, most of it formed in the outer regions of dying stars," said team leader Brice Ménard of the Canadian Institute for Theoretical Astrophysics. "The surprise is that we are seeing dust hundreds of thousands of light-years outside of the galaxies, in intergalactic space."

The new findings are reported in a paper titled "Measuring the galaxy-mass and galaxy-dust correlations through magnification and reddening," submitted to the journal Monthly Notices of the Royal Astronomical Society, and posted today on the web site arXiv.org.

To discover this intergalactic dust, the team analyzed the colors of distant quasars whose light passes in the vicinity of foreground galaxies on its way to the Earth.

Dust grains block blue light more effectively than red light, explained astronomer Ryan Scranton of the University of California, Davis, another member of the discovery team. "We see this when the sun sets: light rays pass through a thicker layer of the atmosphere, absorbing more and more blue light, causing the sun to appear reddened. We find similar reddening of quasars from intergalactic dust, and this reddening extends up to ten times beyond the apparent edges of the galaxies themselves."

The team analyzed the colors of about 100,000 distant quasars located behind 20 million galaxies, using images from SDSS-II. "Putting together and analyzing this huge dataset required cutting-edge ideas from computer science and statistics," said team member Gordon Richards of Drexel University. "Averaging over so many objects allowed us to measure an effect that is much too small to see in any individual quasar."

Supernova explosions and "winds" from massive stars drive gas out of some galaxies, Ménard explained, and this gas may carry dust with it. Alternatively, the dust may be pushed directly by starlight.

"Our findings now provide a reference point for theoretical studies," said Ménard.

Intergalactic dust could also affect planned cosmological experiments that use supernovae to investigate the nature of "dark energy," a mysterious cosmic component responsible for the acceleration of the expansion of the universe.

"Just like household dust, cosmic dust can be a nuisance," said Scranton. "Our results imply that most distant supernovae are seen through a bit of haze, which may affect estimates of their distances."

Intergalactic dust doesn't remove the need for dark energy to explain current supernova data, Ménard explained, but it may complicate the interpretation of future high-precision distance measurements. "These experiments are very ambitious in their goals," said Ménard, "and subtle effects matter."

Astronomers see first quasar trio

Astronomers have found the first example of a triple quasar, the American Astronomical Society meeting in Seattle has heard.Quasars are powerful sources of energy, thought to be powered by supermassive black holes.At first, researchers thought the triplet was just an illusion, caused by the splitting of light beams.But a team using Hawaii's WM Keck Observatory has found the system really involves three black holes. Each quasar produces massive amounts of electromagnetic energy, including visible light and radio waves.They are powered by gas falling into a black hole at the centre of a galaxy. This happens most efficiently when galaxies collide and merge.A single quasar could be a thousand times brighter than an entire galaxy of a hundred billion stars.

Lensing effect

Professor George Djorgovski, from the California Institute of Technology in Pasadena, and colleagues studied a system called LBQS 1429-008.It was found by another group of astronomers in 1989. They reported that it seemed to consist of two quasars.At first, it was thought that one of the quasar pair was a mirage caused by an effect known as gravitational lensing.This effect is caused by an object of large mass getting in the path of light coming from the quasar. This splits the light beams, in essence creating a double image.But astronomers have since identified around 100,000 quasars and dozens of genuine binary quasars.Professor Djorgovski's team found a third, faint quasar using one of the Keck's 10m telescopes and measurements from one of the European Southern Observatory's 8.2m telescopes in Chile.

Galaxy regulation

The team used computer modelling to see if the observations could be easily explained by gravitational lensing. This explanation turned out not to be a good fit.If the triple quasar had been down to lensing, the astronomers should have seen four quasar sources, not three. There would have to be something hiding one of the images.There was no sign of a galaxy, or cluster of galaxies, that could have been the cause of the lensing effect.The team has also documented small, but significant differences in the properties of the three quasars.This observation is much easier to understand if the three quasars are physically distinct objects, rather than mirages."Quasars are extremely rare objects," says Professor Djorgovski. "To find three is unprecedented".Professor Djorgovski thinks that the distribution of quasars in the Universe is not random.Instead, he thinks that the collision and merging of galaxies - and the supermassive black holes that reside at their centres - may actually fuel these powerful sources of energy.This could explain why there are more than the expected number of binary quasars.The quasar is being seen during a period of cosmic time when such interactions between galaxies were at their height.These phenomena may even play an important role in regulating galaxy growth, leading to the joint formation of galaxies and their supermassive black holes - which power the quasars.Professor Djorgovski said it was also possible that quadruple quasars could yet be found.