‘Eye of Sauron’ provides new way of measuring distances to galaxies

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A team of scientists, led by Dr Sebastian Hoenig from the University of Southampton, have developed a new way of measuring precise distances to galaxies tens of millions of light years away, using the W. M. Keck Observatory near the summit of Mauna Kea in Hawaii.

'Eye of Sauron' provides new way of measuring distances to galaxies
The Galaxy NGC 4151 is called the, “Eye of Sauron” due to its similarity to the eye in
 the film Lord of the Rings. The image shows the supermassive black hole, which is still active, 
that is to say that it engulfs gas and dust clouds from its surroundings. In this process, 
it emits ultraviolet radiation, which heats the ring-shaped dust cloud that orbits 
around the black hole at a distance and this causes the dust cloud to emi
t infrared radiation [Credit: NASA]

The method is similar to what land surveyors use on Earth, by measuring the physical and angular, or ‘apparent’, size of a standard ruler in the galaxy, to calibrate the distance from this information.

The research, which is published in the journal Nature, was used to identify the accurate distance of the nearby NGC4151 galaxy, which wasn’t previously available. The galaxy NGC 4151, which is dubbed the ‘Eye of Sauron’ by astronomers for its similarity to the film depiction of the eye of the character in The Lord of the Rings, is important for accurately measuring black hole masses.

Recently reported distances range from 4 to 29 megaparsecs, but using this new method the researchers calculated the distance of 19 megaparsecs to the supermassive black hole.

Indeed, as in the famous saga, a ring plays a crucial role in this new measurement. All big galaxies in the universe host a supermassive black hole in their centre and in about a tenth of all galaxies, these supermassive 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 — becoming the most energetic sources of emission in the universe known as active galactic nuclei (AGN).

'Eye of Sauron' provides new way of measuring distances to galaxies
By combining the light from the two 10-meter Keck telescopes on Mauna Kea 
on Hawaii using a method called interferometry, they could achieve an effective
 resolution equivalent to a telescope with an 85-meter mirror (the distance
 between the two telescopes) [Credit: NASA/JPL]

The hot dust forms a ring around the supermassive black hole and emits infrared radiation, which the researchers used as the ruler. However, the apparent size of this ring is so small that the observations were carried out using infrared interferometry to combine W. M. Keck Observatory’s twin 10-meter telescopes, to achieve the resolution power of an 85m telescope.

To measure the physical size of the dusty ring, the researchers measured the time delay between the emission of light from very close to the black hole and the infrared emission. This delay is the distance the light has to travel (at the speed-of-light) from close to the black hole out to the hot dust.

By combining this physical size of the dust ring with the apparent size measured with the data from the Keck interferometer, the researchers were able to determine a distance to the galaxy NGC 4151.

'Eye of Sauron' provides new way of measuring distances to galaxies
When the gas falls in towards the black hole, it is heated up and emits ultraviolet radiation. 
The ultraviolet radiation heats the ring-shaped dust cloud, which orbits around the black
 hole at a distance and this causes the dust cloud to emit infrared radiation. Using 
telescopes on Earth, we can now measure the time difference between the light from 
the black hole and the light from the dust cloud. The time difference is 30 days 
[Credit: Marie Dyekj?r Eriksen]

Dr Hoenig says: “One of the key findings is that the distance determined in this new fashion is quite precise — with only about 10 per cent uncertainty. In fact, if the current result for NGC 4151 holds for other objects, it can potentially beat any other current methods to reach the same precision to determine distances for remote galaxies directly based on simple geometrical principles. Moreover, it can be readily used on many more sources than the current most precise method.”

“Such distances are key in pinning down the cosmological parameters that characterise our universe or for accurately measuring black hole masses. Indeed, NGC 4151 is a crucial anchor to calibrate various techniques to estimate black hole masses. Our new distance implies that these masses may have been systematically underestimated by 40 per cent.”

Dr Hoenig, together with colleagues in Denmark and Japan, is currently setting up a new program to extend their work to many more AGN. The goal is to establish precise distances to a dozen galaxies in this new way and use them to constrain cosmological parameters to within a few per cent. In combination with other measurements, this will provide a better understanding of the history of expansion of our universe.

Source: University of Southampton [November 26, 2014]

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