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 SPACE.com.
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 arXiv.org, an open-access Web archive for
preprint articles in the fields of physics, math and computer science.
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