In Deep Galaxy Surveys, Astronomers Get a Boost -- From Gravity
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.
This is a color composite image of the Hubble Ultra Deep Field. Green circles mark the locations of candidate galaxies at a redshift of z~8, while higher-redshift candidates are circled in red. The estimated distances to these candidates have not been confirmed spectroscopically. About 20 to 30 percent of these high-z galaxy candidates are very close to foreground galaxies, which is consistent with the prediction that a significant fraction of galaxies at very high redshifts are gravitationally lensed by individual foreground galaxies. This will help as a guide for future observations planned for the James Webb Space Telescope when it is launched. 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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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.
Now that astronomers are aware of the effect, they can use it to their advantage.
"We just need to be aware that we are looking through ‘lenses,'" Yan said. "Take a real magnifying glass as an example: Through the lens we can see more details of an object -- which is a good thing -- but you should be aware that all those details are not actually the sizes that you see, because you're looking through a magnifying lens. The same is true when we're looking at galaxies."
"We predict that many galaxies in the most remote universe will only ever be visible to us because they are magnified in this way," he adds.
Clues From Hubble
Yan is part of an international team of astronomers who are using NASA's Hubble Space Telescope to probe the distant universe. They analyze images from the Hubble Ultra Deep Field (HUDF) survey, a collection of the furthest images of the universe ever taken.
The survey looks back in time 13 billion years, to when the universe was less than one billion years old. Astronomers want to know how many galaxies were bright or faint when the universe was still in that infant stage. So any magnification of those galaxies will interfere with astronomers' ability to judge.
Even through the eyes of Hubble, these faraway galaxies look very small, so it's hard to tell which ones have been magnified. Yet tallies from the HUDF survey are critical to scientists' understanding of how galaxies formed and evolved.
"Although we do not yet have an instrument to directly detect a lensing 'signature' to unambiguously support our prediction, we now have some indirect, tentative evidence that the number of lensed galaxies could be high as we look into the early universe," Yan says. "The apparent association of very distant galaxies to galaxies in the foreground is the key."
Sometimes, the gravitational lensing distorts a galaxy's appearance, or alters its brightness. Other times, the lens splits the light from the faraway galaxy so that two or more galaxies will form around the lens, when there is really only one.
In fact, Yan and his colleagues began this work in order to understand why so many of the faraway galaxies they observed in HUDF survey images appear to be located near the line of sight to galaxies in the foreground.
Through a statistical analysis, they determined that strong gravitational lensing is the most likely explanation.
Yan stressed that the 20-percent estimate is an initial one, and could change in the future.
"We want to make it clear that the size of the effect depends on a number of uncertain factors. If, for example, very distant galaxies are much fainter than their nearby counterparts but much more numerous, the majority of such distant galaxies that we will detect in the foreseeable future could be lensed ones," he says.
According to Yan and his colleagues, the impact of gravitational lensing on galaxy surveys will be even higher in future studies.
Exactly how much is an open question, and Yan says that only the James Webb Space Telescope, set for launch later this decade, could provide a decisive answer.
Yan's discovery suggests that when astronomers use JWST to hunt for faraway galaxies, they should search close to foreground galaxies.
"At Hubble's resolution one literally can no longer see the whole ‘forest for the trees' at these extreme distances. Only the James Webb Space Telescope will have the exquisite resolution and sensitivity to disentangle these very distant objects from the foreground lensing galaxies,'' adds Windhorst.
"This also means that JWST needs to have a very good resolution, so that the galaxies won't just blend together," says Mao.
Funding for this work came from the Australian Research Council, the Space Telescope Science Institute, a NASA-JWST Interdisciplinary Scientist grant, and Yan's fellowship at the Center for Cosmology and Astro-Particle Physics at Ohio State.