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Gravitationally Lensed

Astronomers who survey galaxies in the distant universe are getting some unexpected help from gravity, according to a new study. In a presentation at the American Astronomical Society meeting this week and a related paper in the current issue of the journal Nature, researchers say that as many as 20 percent of the most distant galaxies currently detected appear brighter than they actually are, because of an effect called "strong gravitational lensing." The discovery could change astronomers' notions of how galaxies formed in the early universe. It will also be important in the planning for how to effectively use NASA's planned James Webb Space Telescope (JWST) for hunting very distant galaxies.

Principal investigator Stuart Wyithe of the University of Melbourne calculated the lensing effect at various distances. Wyithe and Haojing Yan, postdoctoral fellow at the Center for Cosmology and Astro-Particle Physics at Ohio State University, collaborated with Rogier Windhorst at Arizona State University and Shude Mao of the University of Manchester and the Chinese Academy of Sciences. Yan reports that astronomers have long known about strong gravitational lensing, but thought it only happens rarely and wouldn't have any real impact on galaxy surveys. "On one hand, lensing is good for us in that it enables us to detect galaxies that would otherwise be invisible; but on the other hand, we will need to correct our surveys to obtain accurate tallies," Yan says.

From our view on Earth, if a faraway galaxy and a nearby galaxy line up on the sky, the gravity of the nearby galaxy bends the light from the faraway galaxy, as if the nearer galaxy were a magnifying glass, or lens. Einstein predicted decades ago that gravity could bend light, and astronomers have since proven him right. In fact, modern astronomers exploit the effect to find distant objects that would otherwise be invisible, such as planets orbiting other stars. And in this case, a statistical analysis revealed that gravitational lensing is brightening faraway galaxies that would otherwise be too faint to see.

Credit: NASA, ESA, S. Wyithe (University of Melbourne), H. Yan (Ohio State University), R. Windhorst (Arizona State University), and S. Mao (Jodrell Bank Center for Astrophysics, and National Astronomical Observatories of China)

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