Stars Living On The Edge
October 25, 2012

Isolated Stars Show Themselves Off To Spitzer

April Flowers for - Your Universe Online

A mysterious infrared glow seen across the entire sky comes from isolated stars beyond the edges of galaxies, according to a new study using data from NASA's Spitzer Space Telescope. The study, published in the journal Nature, suggests that these stars once belonged to the galaxies they edge up to, but were stripped away into the relatively empty space outside during violent galaxy mergers.

Astronomers have disagreed for a long time about why they see more light in the universe than they should. Scientists have seen sandpaper-like smatterings of light in what should have been a uniform dark sky. UCLA's Edward L. (Ned) Wright, professor of physics and astronomy, calls these smatterings "fluctuations" and the debate centers around what the source of those fluctuations is.

Previous theories have suggested that the fluctuations are from very distant and unknown galaxies, or that they are from faint, unknown galaxies that are much closer, whose light has only been traveling to us for 4 or 5 million years. This is a relatively short time in astronomical terms. The new study claims both of these theories are incorrect. The data from Spitzer does not support the distant galaxy theory in the least, said Wright.

"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."

"The infrared background glow in our sky has been a huge mystery," said Asantha Cooray, professor of physics and astronomy at the University of California at Irvine. "We have new evidence this light is from the stars that linger between galaxies. Individually, the stars are too faint to be seen, but we think we are seeing their collective glow."

One opposing theory, developed by Alexander "Sasha" Kashlinsky of NASA´s Goddard Space Flight Center, proposes that this light, which appears in the Spitzer image as a blotchy pattern, comes from the first stars and 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. "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 larger galaxy clusters, astronomers have noticed as much as 20 percent of intrahalo light.

Cooray and his team looked at data from a larger portion of the sky, called the Bootes field. The Bootes field covers an arc equivalent to 50 full Earth moons. The observations from the Kashlinsky study were more sensitive, but the larger scale of the new study allowed researchers to analyze the pattern of the background infrared light better.

"We looked at the Bootes field with Spitzer for 250 hours," said co-author Daniel Stern of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Studying the faint infrared background was one of the core goals of our survey, and we carefully designed the observations in order to directly address the important, challenging question of what causes the background glow."

The light pattern of the infrared glow is inconsistent with current theories and computer models of the first stars and galaxies, according to the research team. The study finds that the glow is too bright to be from the first galaxies, which are thought to be smaller and less numerous than the galaxies around us today. The team has proposed a new theory to explain the blotchy light based on previous theories of "intracluster" or "intrahalo" light.

These theories predict a smattering of stars in the spaces between clusters of galaxies and beyond the halos, or outer reaches, of galaxies. Two phenomena can explain the presence of these stars outside the reaches of their host galaxies. As galaxies grew in size during the early history of our universe, they collided with each other and gained mass. The colliding galaxies became gravitationally tangled and strips of stars were shredded and tossed into space. Larger galaxies also swallow up smaller dwarf galaxies to grow in size, resulting in stray stars as well.

"A light bulb went off when reading some research papers predicting the existence of diffuse stars," Cooray said. "They could explain what we are seeing with Spitzer."

"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 research team says that more research is needed to confirm their theory that this sprinkling of stars makes up a significant fraction of the background infrared light. They say it would be necessary to find a similar pattern in follow-up observations of visible light, and the hope is that NASA's upcoming James Webb Space Telescope (JWST) will settle the matter for good.

"The keen infrared vision of the James Webb Telescope will be able to see some of the earliest stars and galaxies directly, as well as the stray stars lurking between the outskirts of nearby galaxies," said Eric Smith, JWST's deputy program manager at NASA Headquarters in Washington. "The mystery objects making up the background infrared light may finally be exposed."

Image 2 (below): 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). Image credit: NASA/JPL-Caltech