Astronomers have found the most distant gravitational lens yet--a galaxy that, as predicted by Albert Einstein's general theory of relativity, deflects and intensifies the light of an even more distant object.

The discovery provides a rare opportunity to directly measure the mass of a distant galaxy. But it also poses a mystery, because lenses of this kind should be exceedingly rare. Given this and other recent finds, astronomers either have been phenomenally lucky or, more likely, they have underestimated the number of small, very young galaxies in the early universe.

"It's like stumbling across a gold nugget while hiking in the Sierras," said David Koo, professor of astronomy and astrophysics at UC Santa Cruz and coauthor of a paper on the discovery published in Astrophysical Journal Letters. "People believe such gold nuggets are too rare to be found by chance, but perhaps there's actually a lot more 'gold in them thar hills.'"

Arjen van der Wel from the Max Planck Institute for Astronomy (MPIA) led the team that made the discovery, which also includes UC Santa Cruz astronomer Sandra Faber. Faber leads the CANDELS survey, which provided images of the distant universe used in the study. Koo is a co-founder of the CANDELS survey and has been studying gravitational lenses since the 1980s.

Light is affected by gravity, and light passing a distant galaxy will be deflected as a result. Since the first find in 1979, numerous such gravitational lenses have been discovered. In addition to providing tests of Einstein's theory of general relativity, gravitational lenses have proved to be valuable tools. Astronomers can determine the mass of the matter that is bending the light, including the mass of dark matter, which does not emit or absorb light and can only be detected via its gravity. Also, the lens magnifies the background light source, acting as a "natural telescope" that allows astronomers a more detailed look at distant galaxies than is normally possible.

Gravitational lenses consist of two objects: one that is more distant and another, the lensing mass or gravitational lens, which sits between us and the distant light source and whose gravity deflects the light from the farther object. When the observer, the lens, and the distant light source are precisely aligned, the observer sees an "Einstein ring," a perfect circle of light that is the projected and greatly magnified image of the distant light source.

The new discovery is the most distant gravitational lens yet found. "The discovery was completely by chance," said van der Wel. "I had been reviewing observations from an earlier project with the goal of measuring masses of old, distant galaxies by looking at the motion of their stars. Among the galaxy spectra [the rainbow-like split of a galaxy's light into different wavelengths], I noticed a galaxy that was decidedly odd. It looked like an extremely young galaxy, and at an even larger distance than I was aiming for. It shouldn't even have been part of our observing program."

Van der Wel followed up the spectra, which were taken with the Large Binocular Telescope in Arizona, by looking at images taken with the Hubble Space Telescope as part of the CANDELS and COSMOS surveys. The object looked like an old galaxy, a plausible target for the original observing program, but with some irregular features which, he suspected, meant that he was looking at a gravitational lens. Combining the available images and removing the haze of the lensing galaxy's collection of stars, the result was very clear: an almost perfect Einstein ring, indicating a gravitational lens with very precise alignment of the lens and the background light source.

The lensing mass is so distant that the light, after having been deflected, has traveled 9.4 billion years to reach us (the total age of the universe is 13.8 billion years). The previous record holder, found thirty years ago, took less than 8 billion light-years for its light to reach us.

Not only is this a new record, the object also serves an important purpose: The amount of distortion caused by the lensing galaxy allows for a direct measurement of its mass. This provides an independent test for astronomers' usual methods of estimating distant galaxy masses, which rely on extrapolation from their nearby cousins. Fortunately for astronomers, their usual methods pass the test.

But the discovery also poses a puzzle because it is so unlikely. Gravitational lenses are the result of a chance alignment. In this case, the alignment is very precise. In addition, the magnified object is a so-called "star-bursting dwarf galaxy," a comparatively light galaxy (only about 100 million solar masses worth of stars), but extremely young (about 10 to 40 million years old) and producing new stars at an enormous rate. The chances for such peculiar galaxies to be gravitationally lensed are very small. Yet this is the second star-bursting dwarf galaxy found to be lensed. If such galaxies are much more common than previously thought, astronomers may be forced to rethink their models of galaxy evolution.

"I have been studying the mystery of distant, tiny, bright-blue baby galaxies for nearly three decades, and this makes the mystery even deeper," Koo said. "Are such baby galaxies more common than current theories predict? If they are so common early in the history of the universe, what happens to them? Are they the ancient progenitors of the now faint red tiny galaxies swirling around our Milky Way galaxy? Or are they perhaps the progenitors of galaxies like the Milky Way?"

The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) is a powerful survey of the distant universe being carried out with the Hubble Space Telescope. The largest project in the history of Hubble, it has been allocated observing time amounting to 900 of the space telescope's orbits around Earth. Taken together with other observations covering the same region, the CANDELS researchers can use what amounts to a combined exposure time of nearly 4 months of Hubble data. CANDELS uses two instruments aboard the HST: the near-infrared WFC3 camera and the visible-light ACS camera. Jointly, these two cameras give unprecedented coverage of galaxies from optical wavelengths to the near-infrared. This will allow CANDELS to study different stages in the formation of galaxies, from the first billion years of cosmic evolution to the present.