Gravitational lensing occurs when light from a distant object is distorted by a massive object that is in the foreground. Astronomers have started to apply this concept in a new way to determine the number of very distant galaxies and to measure dark matter in the universe. Though recent progress has been made in extending the use of gravitational lensing, a Letter published in Nature on January 12, 2011, by Stuart Wyithe (U. Melbourne),Haojing Yan (OSU), Rogier Windhorst (ASU), and Shude Mao (NAOC, China and Jodrell Bank, UK) makes the case that the tool may be even more necessary than originally thought when observing distant galaxies.

In the 1930's Albert Einstein showed that gravity will cause light to bend. The effect is normally extremely small, but when light passes close to a very massive object -- a "gravitational lens" as astronomers would call it -- the bending of the light rays becomes more easily noticeable. When light from a very distant galaxy passes a massive object much closer to us, it can detour around this foreground "lens" and forms a larger and brighter image or even multiple images of the distant galaxy. In this case, the foreground object works as a "natural telescope" and makes the distant galaxy more easily detectable to us (see Figure 1 for illustrations).

Figure 1

This effect is not new to astronomers, and in fact a few massive clusters of galaxies have been used as such "natural telescopes" in the past to search for very distant galaxies. Mao explains, "The lensing magnification gives a better chance to detect distant objects in the universe otherwise too faint to see, and with much finer spatial details." Individual galaxies, while much less massive than galaxy clusters, can also work as lenses as well if they are heavy and dense enough. The chance of multiple images is one in a few hundred for background sources such as quasars. What Mao and his collaborators have found is that the incidence of gravitational lensing for very by individual foreground galaxies might be higher than what people had thought for objects at the very early universe.

When we look back to when the universe was young, we are seeing extremely early objects that are very far away. The farther away the object, the more of the foreground universe there is to look through, which means the greater the chance that there will be something heavy in the foreground that case be used as a lens to magnify the background image. With its new Wide Field Camera 3 (WFC3), the Hubble Space Telescope (HST) has begun to see some of the brightest galaxies at redshifts of 8-10, which correspond to a very early stage of the universe when it was only 650-480 million years old. The future James Webb Space Telescope (JWST) will not only enable us to detect many more fainter galaxies at these redshifts but also galaxies at even higher redshifts. A future generation of large telescopes, such as the Thirty Meter Telescope (TMT, of which China is now in the early stage of joining), will allow us to characterize the stellar populations and internal dynamics in these very distant galaxies.

In their Nature Letter, Mao and his colleagues suggest that our measurement of the flux and count of very distant galaxies is likely to be significantly distorted by the magnification through individual foreground galaxies. The exact ratio of magnified and unmagnified distant galaxies depends on a number of factors which astronomers are currently not certain about. After exploring a large number of possibilities, the team finds that the fraction of lensed galaxies ranges from a few percent to a few tens of percent. In extreme cases, all that we will be able to see at redshift of 20 with the future JWST would be lens-magnified objects. "This will be an important effect to take into account when interpreting the future survey data." Mao explains.

In fact, there might already have been some evidence that we are seeing such a high incidence of lensing events in the recent deep near-IR surveys by the HST WFC3. In a separate study one year ago, a team led by Yan, a co-author of this Nature Letter, used these data and found a large number of candidate galaxies at redshifts of 8--10. Their result was published in the journal Research in Astronomy and Astrophysics (at www.raa-journal.org based at NAOC) in September 2010. Puzzlingly, around 20-30% of their candidate distant galaxies (examples shown in the circled regions in Figure 2) are found around foreground galaxies which cannot simply be attributed to alignment-by-chance. The new study shows that the observed, unexpectedly high rate of concentration is consistent with the prediction from gravitational lensing. However, the evidence from HST so far is still only tentative, and more powerful telescopes such as JWST and TMT will be needed to provide definitive answers from the increased sensitivity both in imaging and spectroscopic observations. "Along the way, these future generation instruments will provide us a much more detailed picture of how structures such as our Milky are pieced together from the early universe." says Mao.

Figure 2

Editor’s Caption on the Letter published in Nature on January 12, 2011:
Gravitational Lens
Einstein's Theory of General Relativity predicts that light bends about massive objects, a phenomenon called a Gravitational Lens. When observing distant galaxies in the Universe, astronomers can use massive galaxies as gravitational lenses to observe even more distant galaxies, which would otherwise be too faint for astronomers to see. This Nature Letter highlights some recent findings that show gravitational lenses are more common than previously believed, especially when observed by the upcoming next generation of large telescopes, such as the Thirty-Meter Telescope and the James Webb Space Telescope. Using such gravitational lenses will allow astronomers to study the earliest and most distant galaxies in our Universe.