Astronomers report dark matter 'halos' may contain stars, disprove other theories
Unmasking a hidden glow
The image on the left shows a portion of our sky, called the Boötes field, in infrared light,
while the image on the right shows a mysterious, background infrared glow captured by NASA's Spitzer Space Telescope
in the same region of sky. Using Spitzer, researchers were able to detect this background glow, which spreads across
the whole sky, by masking out light from galaxies and other known sources of light. (The masks are the gray, blotchy marks).
The scientists find that this light is coming from stray stars that were torn away from galaxies. When galaxies tangle and merge,
stars often get kicked out in the process. The stars are too faint to be seen individually, but Spitzer may be seeing their
(Image credit: NASA/JPL-Caltech/UC Irvine)
October 24, 2012
By Stuart Wolpert
Could it be that dark matter "halos" — the huge, invisible cocoons of mass that envelop entire galaxies and account for most
of the matter in the universe — aren't completely dark after all but contain a small number of stars? Astronomers from UCLA,
UC Irvine and elsewhere make a case for that in the Oct. 25 issue of the journal Nature.
Astronomers have long disagreed about why they see more light in the universe than it seems they should — that is,
why the infrared light they observe exceeds the amount of light emitted from known galaxies.
When looking at the cosmos, astronomers have seen what are neither stars nor galaxies nor a uniform dark sky but mysterious,
sandpaper-like smatterings of light, which UCLA's Edward L. (Ned) Wright refers to as "fluctuations." The debate has centered
around what exactly the source of those fluctuations is.
One explanation is that the fluctuations in the background are from very distant unknown galaxies. A second is that they're from
unknown galaxies that are not so far away, faint galaxies whose light has been traveling to us for only 4 billion or 5 billion
years (a rather short time in astronomy terms). In the Nature paper, Wright and his colleagues present evidence that both these
explanations are wrong, and they propose an alternative.
The first explanation — that the fluctuations are from very distant galaxies — is nowhere close to being supported by the data
the astronomers present from NASA's Spitzer Space Telescope, said Wright, a UCLA professor of physics and astronomy.
"The idea of not-so-far-away faint galaxies is better, but still not right," he added. "It's off by a factor of about 10;
the 'distant galaxies' hypothesis is off by a factor of about 1,000."
Wright and his colleagues, including lead author Asantha Cooray, a UC Irvine professor of physics and astronomy, contend that
the small number of stars that were kicked to the edges of space during violent collisions and mergers of galaxies may be the
cause of the infrared light "halos" across the sky and may explain the mystery of the excess emitted infrared light.
As crashing galaxies became gravitationally tangled with one another, "orphaned" stars were tossed into space. It is these stars,
the researchers say, that produce the diffuse, blotchy scatterings of light emitted from the galaxy halos that extend well beyond
the outer reaches of galaxies.
"Galaxies exist in dark matter halos that are much bigger than the galaxies; when galaxies form and merge together, the dark matter
halo gets larger and the stars and gas sink to the middle of the halo," said Wright, who holds UCLA's David Saxon Presidential
Chair in Physics. "What we're saying is one star in a thousand does not do that and instead gets distributed like dark matter.
You can't see the dark matter very well, but we are proposing that it actually has a few stars in it — only one-tenth of 1 percent
of the number of stars in the bright part of the galaxy. One star in a thousand gets stripped out of the visible galaxy and gets
distributed like the dark matter.
"The dark matter halo is not totally dark," Wright said. "A tiny fraction, one-tenth of a percent, of the stars in the central
galaxy has been spread out into the halo, and this can produce the fluctuations that we see."
In large clusters of galaxies, astronomers have found much higher percentages of intra-halo light, as large as 20 percent, Wright
For this study, Cooray, Wright and colleagues used the Spitzer Space Telescope to produce an infrared map of a region of the sky
in the constellation Boötes. The light has been travelling to us for 10 billion years.
"Presumably this light in halos occurs everywhere in the sky and just has not been measured anywhere else," said Wright, who is
also principal investigator of NASA's
Wide-field Infrared Survey Explorer
"If we can really understand the origin of the infrared background, we can understand when all of the light in the universe was
produced and how much was produced," Wright said. "The history of all the production of light in the universe is encoded in this
background. We're saying the fluctuations can be produced by the fuzzy edges of galaxies that existed at the same time that most
of the stars were created, about 10 billion years ago."
The research was funded by the National Science Foundation, NASA and NASA's Jet Propulsion Laboratory.
Future research, especially with the
James Webb Space Telescope,
should provide further insights, Wright said.
"What we really need to be able to do is to see and identify the galaxies that are producing all the light in the infrared
background," he said. "That could be done to a much greater extent once the James Webb Space Telescope is operational because it
will be able to see much more distant, fainter galaxies."
Co-authors on the Nature paper include Daniel Stern of NASA's Jet Propulsion Laboratory in Pasadena, Calif.;
UC Irvine doctoral student Joseph Smidt; Francesco De Bernardis, Yan Gong and Christopher Frazer of UC Irvine;
Peter Eisenhardt of NASA's JPL; Matthew Ashby of the Harvard–Smithsonian Center for Astrophysics; Anthony Gonzalez
of the University of Florida; Christopher Kochanek of Ohio State University; and Szymon Kozłowski of Ohio State University
and the Warsaw University Astronomical Observatory.
JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate in Washington. Science operations are
conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Data are archived at the Infrared
Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA. For more
information about Spitzer, visit