Looking Up – Bob Eklund

n4151_300_300_sA team of scientists, led by Dr. Sebastian Hoenig from the University of Southampton, has measured the distance to the NGC 4151 (“Eye of Sauron”) galaxy with unprecedented accuracy, using the W. M. Keck Observatory Interferometer in Hawaii.

The team employed a new technique they developed, which allows them to measure distances to galaxies tens of millions of light-years away with 90 percent precision. The technique is similar to that used by land surveyors on Earth, who calculate distances by measuring the angular—or “apparent”—size of an object of known dimensions.

Previous reported distances to NGC 4151, which contains a supermassive black hole, were far from consistent—ranging all the way from 4 to 29 megaparsecs. Using this new, more accurate method, the researchers calculated the distance to the supermassive black hole as 19 megaparsecs (a megaparsec is equal to 3.26 million light-years).

Galaxy NGC 4151 is dubbed the “Eye of Sauron” by astronomers for the similarity of its appearance to its namesake in the film trilogy, “The Lord of the Rings.” As in that saga, a “ring” plays a crucial role in this new measurement. All big galaxies in the universe host a supermassive black hole in their center—and in about 10 percent of all galaxies, these black holes are growing by swallowing huge amounts of gas and dust from their surrounding environments. In this process, the material heats up and becomes very bright—resulting in the most energetic sources of emission in the universe, known as active galactic nuclei (AGN).

This hot dust forms a ring around the supermassive black hole and emits infrared radiation—and the researchers in this study used this dusty ring as their “ruler.” However, the apparent size of the Eye of Sauron’s ring is so small that the observations had to be carried out using the Keck Interferometer, which combines Keck Observatory’s twin 10-meter telescopes—already the largest telescopes on Earth—to achieve a resolving power equal to that of an 85-meter telescope.

To measure the physical size of the dusty ring, the researchers measured the time delay between the emission of light from close to the black hole and the more distant infrared emission. The distance from the center to the hot dust is simply this delay divided by the speed of light.

By combining the physical size of the dust ring with the apparent size measured with the Keck Interferometer, the researchers were able to determine the distance to NGC 4151.

“One of the key findings is that the distance determined in this new fashion is quite precise—with 90 percent accuracy,” Hoenig said. “In fact, this method, based on simple geometrical principles, gives the most precise distances for remote galaxies. Moreover, it can be readily used on many more sources than current methods. Such distances are key in pinning down the cosmological parameters that characterize our universe or in accurately measuring black hole masses. Indeed, NGC 4151 is a key to calibrating various techniques of estimating black hole masses. Our new distance implies that these masses may have been systematically underestimated by 40 percent.”

Hoenig, together with colleagues in Denmark and Japan, is currently setting up a new program to extend their work to many more active galactic nuclei. The goal is to establish precise distances to a dozen galaxies using this technique—and then use these precise distances as stepping-stones to vastly improve the precision of all distance measurements for the cosmos.

Photo credit – NASA


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