Determining the distance of galaxies from our Solar System is a tricky business. Knowing just how far other galaxies are in relation to our own is not only key to understanding the size of the universe, but its age as well. In the past, this process relied on finding stars in other galaxies whose absolute light output was measurable. By gauging the brightness of these stars, scientists have been able to survey certain galaxies that lie 300 million light years from us.
However, a new and more accurate method has been developed, thanks to a team of scientists led by Dr. Sebastian Hoenig from the University of Southampton. Similar to what land surveyors use here on Earth, they measured the physical and angular (or apparent) size of a standard ruler in the galaxy to calibrate distance measurements.
Hoenig and his team used this method at the W. M. Keck Observatory, near the summit of Mauna Kea in Hawaii, to accurately determine for the first time the distance to the NGC 4151 galaxy – otherwise known to astronomers as the “Eye of Sauron”.The galaxy NGC 4151, which is dubbed the “Eye of Sauron” by astronomers for its similarity to the depiction of Sauron in “The Lord of the Rings” trilogy, 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 a 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. Scientists have observed that all big galaxies in the universe have a supermassive black hole in their center. And in about a tenth of all galaxies, these supermassive black holes continue to grow 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).
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 the distance to the galaxy NGC 4151.
As Dr. Hoenig said: “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 characterize our universe or for accurately measuring black hole masses,” he added. “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.
The research was published on Wednesday, Nov. 26th in the online edition of the journal Nature.