Seeing further with Quasars

A Quasar has been seen acting as a gravitational lens for the first time. Quasars, or Quasi Stellar Objects, are supermassive black holes in the cores of early galaxies believed to be in the process of consuming excess amounts of primordial gas. The process of accretion leads to a variety of emissions so bright they can be seen in the present time. As they looked initially rather like stars, but with high redshifts in their light suggesting they were far away (as the expansion of the universe means the farther away you are, the faster you are moving away and so the more your light, to an observer on the Earth, is Doppler shifted to longer wavelengths).

Gravitational lensing was first described by Einstein in his paper on General Relativity and shown experimentally by Eddington in his 1919 eclipse trip to Principe. Any mass in space causes the fabric of space-time to warp. The greater the mass, the more the warping (which we perceive as gravity). Light has to negotiate these warps and in doing so gets spread out like light seen through the base of a wineglass. This creates a lensing effect whereby things closer to the mass in the sky are seen to apparently shift outwards, with the effect lessening with distance from the mass. In the 1919 trip, Eddington photographed bright stars in the Hyades cluster shift their position away from the Sun, when the Sun was eclipsed.

In the case of what is known as strong gravitational lensing, galaxies and galaxy clusters are seen to lens the light from other galaxies behind them, forming rings or partial rings of light called Einstein rings. This was first demonstrated with a galaxy lensing a bright quasar. Doing this the other way round, however, proves a little more difficult. Firstly, quasars being very distant have fewer objects in the observable universe behind them – and these objects are all going to be rather dim. Secondly, quasars, as mentioned earlier, are very intrinsically bright. They outshine their host galaxies by many orders of magnitude, never mind the dim ring of another galaxy behind them.

The researchers decided to persevere anyway. They looked at the spectra of 20,000 quasars and examined them for the presence of additional spectral lines (identifiable fingerprints of known materials) at even higher redshifts than the quasar. This would indicate the presence of a lens that is bright enough to be visible. This cut the sample down to around a dozen objects. Then the quasars were observed with the Keck II telescope, which has special adaptive optics that can remove the effect of atmospheric turbulence, allowing a cleaner image to be put out.

The image and more on the story, can be seen at Discovery and also at Science Blogs, who also have a post on the 1919 eclipse results.


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