How strong gravitational lensing works; 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, their statistical analysis revealed that gravitational lensing is brightening faraway galaxies that would otherwise be too faint to see.
Haojing Yan, postdoctoral fellow at the Center for Cosmology and Astro-Particle Physics at Ohio State University, presented the information in a presentation at the American Astronomical Society meeting this week and stated 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.
This graphic illustrates how, when astronomers view distant galaxies in a telescope (upper left panel), some of those galaxies line up with our view of nearby galaxies (center panel). The gravity of the nearby galaxies bends and magnifies the light coming from some of those distant galaxies, so that they appear brighter than they actually are (lower right panel). Thus galaxies that would normally be too faint to detect become visible in telescope images. Credit: Artwork : NASA, ESA, and A. Feild (STScI)Science : 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)
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 you 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 added.
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 said. "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 said.
According 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 the only the upcoming James Webb Space Telescope (JWST) could provide a decisive answer, assuming it gets completed in our lifetimes.
Yan's discovery suggests that when astronomers use JWST to hunt for faraway galaxies, they should search close to foreground galaxies.
"This also means that JWST needs to have a very good resolution, so that the galaxies won't just blend together," he said.