Washington: 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, astronomers suggest.
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 may contain a small number of stars, a new study by University of California, Los Angeles and the University of California, Irvine found.
Scientists 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 Edward L (Ned) Wright from UCLA 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 travelling to us for only 4 billion or 5 billion years (a rather short time in astronomy terms).
Researchers present evidence that both these explanations are wrong, and 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 and principal investigator of NASA's Wide-field Infrared Survey Explorer (WISE) mission.
Wright and his colleagues, including lead author Asantha Cooray found that as crashing galaxies became gravitationally tangled with one another, "orphaned" stars were tossed into space.
It is these stars, the researchers said, 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," Wright said in a statement.
"The dark matter halo is not totally dark. A tiny fraction, one-tenth of a per cent, of the stars in the central galaxy has been spread out into the halo, and this can produce the fluctuations that we see," Wright said.
Researchers used the Spitzer Space Telescope to produce an infrared map of a region of the sky in the constellation Bootes.
"Presumably this light in halos occurs everywhere in the sky and just has not been measured anywhere else," said Wright.
The study was published in the journal Nature.
First Published: Thursday, October 25, 2012, 19:20