Space Dust More Pervasive Than Thought
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: Ohio State University

The vast spaces between galaxies might seem pretty empty. But they are actually littered with clouds of cosmic dust that were likely ejected from the galaxies themselves. And the dust scatters farther into intergalactic space than astronomers expected, a new study finds.

The discovery was made by watching subtle shifts in the light emanating from radio sources that sit at the hearts of far-away galaxies.

These radio sources are called quasars, and are the most distant and powerful sources of energy in the universe. From their nests at the center of galaxies, they emit powerful jets of radio waves, X-rays and sometimes high-energy gamma rays. Like celestial flashlights, their light can penetrate through the dark expanse of space.

On its way to Earth, the light from quasars passes by and through intervening galaxies. Dust grains in the galaxies block the light from the blue end of the spectrum more effectively than red light, causing a quasar to appear redder to viewers on Earth.

This same phenomenon can be seen on Earth during a sunset: "Light rays pass through a thicker layer of the atmosphere, absorbing more and more blue light, causing the sun to appear reddened," said Ryan Scranton of the University of California, Davis, who was part of the team that made the dust discovery.

"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," Scranton explained.

The team analyzed the colors of about 100,000 distant quasars located behind 20 million galaxies, using images from the Sloan Digital Sky Survey (SDSS-II).

"Averaging over so many objects allowed us to measure an effect that is much too small to see in any individual quasar," said team member Gordon Richards of Drexel University in Philadelphia.

The light shifts showed that dust wasn't just in the galaxies, it was also outside of them.

"Galaxies contain lots of dust, most of it formed in the outer regions of dying stars," said team leader Brice Menard 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."

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

"Somehow, some of this dust is getting ejected into the space between the galaxies," Richards told

Astronomers had thought that any dust spewed out by exploding stars would fall back into the galaxy, like a baseball falls back to Earth when it's tossed in the air. The dusty material must be jettisoned much faster than scientists had thought to overcome that gravitational pull, just as a rocket must be launched at very high speeds to reach space, Richards explained.

So the new finding means that astronomers will have to "start tweaking theoretical models," to see if they can come up with a mechanism that explains the dust clouds surrounding the galaxies.

This intergalactic dust could 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," Scranton said. "Our results imply that most distant supernovae are seen through a bit of haze, which may affect estimates of their distances."

The findings were submitted to the journal Monthly Notices of the Royal Astronomical Society and posted on Feb. 26 to, an open-access Web archive for preprint articles in the fields of physics, math and computer science.