18 October 2013

Distant galaxy poses astronomical puzzle

by Will Parker

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. However, the discovery poses a mystery: lenses of this kind should be exceedingly rare, so astronomers either have been phenomenally lucky in finding this and other lenses, or, a re-think of our models of galactic evolution is required.

Gravitational lenses act as a "natural telescope" that allows astronomers a more detailed look at distant galaxies than is normally possible. They consist of two objects: one is further away and supplies the light, and the other, the gravitational lens, sits between us and the distant light source. The gravity of the closer object deflects the light and 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. Since the first galactic lens was identified in 1979 numerous other lenses have been discovered.

Now, astronomers have found the most distant gravitational lens yet. "The discovery was completely by chance. I had been reviewing observations from an earlier project when I noticed a galaxy that was decidedly odd. It looked like an extremely young galaxy, but it seemed to be at a much larger distance than expected. It shouldn't even have been part of our observing program!" explained Arjen van der Wel, from the Max Planck Institute for Astronomy.

Van der Wel wanted to find out more and started to study images taken with the Hubble Space Telescope. In these pictures the mystery 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 deflection, has travelled 9.4 billion years 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 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. Gravitational lenses are the result of a chance alignment. In this case, the alignment is very precise. To make matters worse, the magnified object is a starbursting dwarf galaxy: a comparatively light galaxy (it has only about 100 million solar masses in the form of stars), but extremely young (about 10-40 million years old) and producing new stars at an enormous rate. The chances that such a peculiar and supposedly rare galaxy would be gravitationally lensed are infinitesimally small. Yet this is the second starbursting dwarf galaxy that has been found to be lensed. Either astronomers have been phenomenally lucky, or starbursting dwarf galaxies are much more common than previously thought, forcing astronomers to re-think their models of galaxy evolution.

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Source: Hubble Information Centre