Dark matter ‘halos’ may not actually be dark, but composed of small numbers of extragalactic stars, says new research. Dark matter halos are the enormous, ‘invisible’ masses that surround whole galaxies and are estimated to contain most of the matter in the universe.
It’s long been a matter of debate among researchers why there is more observable light in the universe than “it seems they should.” Specifically, why the observable infrared light greatly exceeds the total light that is released from observable 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 of the main theories has been that the ‘light in the background’ is from very distantly located and unknown galaxies. The other main theory has been that the light is coming from unknown galaxies located not very far away, but that are rather faint. ”
In the new research, Wright and his coworkers provide solid evidence that these explanations are both inaccurate. And they provide a better theory to replace those.
“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.”
According to the researchers, the light is created by small numbers of extragalactic stars. Stars that were sent to the edges of space by the gravitational forces generated during violent galactic mergers and collisions. These stars wouldn’t be visible on their own but could create the observed ‘light in the background’.
When galaxies begin merging together, becoming gravitationally tangled, it’s thought that ‘orphan’ stars are often sent 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 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.”
The researchers observed that large clusters of galaxies possess much higher percentages of intra-halo light than others.
So the researchers then used the Spitzer Space Telescope to create 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 (WISE) mission.
“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 researchers note that when the James Webb Telescope is operational, it’ll allow research that could reinforce their work.
“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.”
The new research was just published October 25th in the journal Nature.
Image Credits: NASA/JPL-Caltech/UC Irvine