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Dark matter halos may contain stars

LOS ANGELES and IRVINE, Calif. – Stars kicked to the edges of space during violent collisions and mergers with other galaxies can get tossed into large, invisible cocoons of dark matter, which might explain why astronomers say they see more light in the universe than it seems they should.

The findings suggest that the halos of dark matter surrounding galaxies are not completely dark after all but contain a small number of stars, according to UCLA and University of California, Irvine, scientists.

Satellites have detected more infrared light radiating from the sky than known galaxies could account for. Scientists have suggested that this mysterious, sandpaperlike smattering of light comes from sources too dim for observatories to see directly – i.e., from very distant unknown galaxies. A second hypothesis credits the emanating light to 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.

“The idea of not-so-far-away faint galaxies is better, but still not right,” said Edward L. “Ned” Wright, a UCLA professor of physics and astronomy. “It’s off by a factor of about 10; the ‘distant galaxies’ hypothesis is off by a factor of about 1000.”


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 detected 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 say that this light is coming from stray stars torn 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 collective glow.


Now, using NASA’s Spitzer Space Telescope, Wright and colleagues have produced an infrared map of a region of the sky in the constellation Boötes. The light has been traveling to us for 10 billion years. The scientists contend that the small number of stars that were kicked to the edges of space 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 entangled with one another, “orphaned” stars were tossed into space. It is these stars, the researchers say, that produce the diffuse, blotchy scatterings of light – dubbed fluctuations by Wright – 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,” Wright said. “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.”

Much higher percentages of intra-halo light, as large as 20 percent, have been found in large clusters of galaxies, he said.

“Presumably, this light in halos occurs everywhere in the sky and just has not been measured anywhere else,” Wright said. “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.”

Research with the James Webb Space Telescope once it is in operation should provide further insight because it will be able to see much more distant, fainter galaxies, he said.

The findings were reported in Nature (doi: 10.1038/nature11474).


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