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Galaxy halos are produced by orphan stars, findings indicate

UCI, other researchers discount previous theories about origins of infrared glow
Isolated stars kicked to the edges of space by violent galaxy mergers may be the cause of mysterious infrared light halos observed across the sky, according to UC Irvine and other astronomers.

“Background glow in our sky has been a huge unanswered question,” said UCI physics & astronomy professor Asantha Cooray, lead author of a paper about the discovery in the Oct. 25 issue of the journal Nature. “We have new evidence that this light is from stars that linger between galaxies. Individually, they’re too dim to be seen, but we think we’re seeing their collective blush.”

Cooray and colleagues examined 250 hours of data captured by NASA’s powerful Spitzer Space Telescope from a large swath of sky called the Boötes field, which covers the equivalent of 40 full moons near the constellation of the same name. The large scale allowed the researchers to better analyze the patterns of diffuse light.

“Studying this faint background was one of the core goals of our survey, and we carefully designed the observations in order to directly address this important, challenging question,” said co-author Daniel Stern of NASA’s Jet Propulsion Laboratory in Pasadena.

The team concluded that the infrared glow, while weak, is too strong to be consistent with earlier theories that it’s being emitted by the very first celestial bodies. “The glow is just too bright to be from those ancient, far-off galaxies and stars,” said UCI doctoral student and co-author Joseph Smidt.

Instead, the scientists have a new theory, saying it’s “intracluster” or “intrahalo” starlight. Early in the history of the universe, as galaxies grew, they collided and bulked up in mass. As the crashing galaxies became gravitationally tangled, strips of stars were shredded and tossed into space as leftovers. Galaxies also grow by “swallowing” dwarf neighbors, a messy process that likewise results in stray stars. Cosmologists believe these orphaned stars produce the diffuse, blotchy smatterings of light that make up galaxy halos extending well beyond the outer reaches of galaxies.

Additional research is needed to confirm the theory. But the researchers say it makes sense. “A lightbulb went off when we were reading earlier papers predicting the existence of diffuse stars,” Cooray said. “They explain what we’re seeing with Spitzer.”

Other authors are UCI postdoctoral students Francesco De Bernardis and Yan Gong and undergraduate Christopher Frazer; Peter Eisenhardt of NASA’s Jet Propulsion Laboratory; Matthew Ashby of the Harvard-Smithsonian Center for Astrophysics; Anthony Gonzalez of the University of Florida; Christopher Kochanek and Szymon Koz³owski of The Ohio State University (Koz³owski is also with the Warsaw University Astronomical Observatory); and Edward “Ned” Wright of UCLA. Funding was provided by the National Science Foundation, NASA and JPL.

About the University of California, Irvine: Founded in 1965, UCI is a top-ranked university dedicated to research, scholarship and community service. Led by Chancellor Michael Drake since 2005, UCI is among the most dynamic campuses in the University of California system, with nearly 28,000 undergraduate and graduate students, 1,100 faculty and 9,000 staff. Orange County’s second-largest employer, UCI contributes an annual economic impact of $4 billion. For more UCI news, visit

News Radio: UCI maintains on campus an ISDN line for conducting interviews with its faculty and experts. Use of this line is available for a fee to radio news programs/stations that wish to interview UCI faculty and experts. Use of the ISDN line is subject to availability and approval by the university.

Janet Wilson | EurekAlert!
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