The vast expanses of intergalactic space appear to be filled with ahaze of tiny, smoke-like "dust" particles that dim the light fromdistant objects and subtly change their colors, according to a team ofastronomers from the Sloan Digital Sky Survey (SDSS-II).
"Galaxies contain lots of dust, most of it formed in the outer regionsof dying stars," said team leader Brice Menard of the CanadianInstitute for Theoretical Astrophysics. "The surprise is that we are seeingdust hundreds of thousands of light-years outside of the galaxies, inintergalactic space."
The new findings are reported in a paper titled "Measuring thegalaxy-mass and galaxy-dust correlations through magnification andreddening," submitted to the journal Monthly Notices of the RoyalAstronomical Society, and posted today on the web site arXiv.org.
To discover this intergalactic dust, the team analyzed the colors ofdistant quasars whose light passes in the vicinity of foregroundgalaxies 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 sunsets: light rays pass through a thicker layer of the atmosphere,absorbing more and more blue light, causing the sun to appearreddened."
"We find similar reddening of quasars from intergalactic dust, and thisreddening extends up to ten times beyond the apparent edges ofthe galaxies themselves," said Scranton.
The team analyzed the colors of about 100,000 distant quasars locatedbehind 20 million galaxies, using images from SDSS-II. "Putting togetherand analyzing this huge dataset required cutting-edge ideas from computerscience and statistics," said team member Gordon Richards of DrexelUniversity. "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 ofsome galaxies, Menard 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 Menard.
Intergalactic dust could also affect planned cosmologicalexperiments that use supernovae to investigate the nature of"dark energy," a mysterious cosmic component responsible forthe 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 areseen through a bit of haze, which may affect estimates of their distances."
Intergalactic dust doesn't remove the need for dark energy to explaincurrent supernova data, Menard explained, but it may complicate theinterpretation of future high-precision distance measurements."These experiments are very ambitious in their goals," said Menard, "andsubtle effects matter."
Source: Sloan Digital Sky Survey
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. 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.
(Photo Credit: The Sloan Digital Sky Survey)