Free Astronomy Magazine January-February 2015

SPACE CHRONICLES T his image shows the spiral galaxy NGC 4151, lo- cated at a dis- tance of about 45 million light years from us. NGC 4151 is a Seyfert galaxy and hosts one of the brightest active galactic nuclei (AGN) known at X-ray wavelengths. The supermas- sive black hole lying at the cen- tre of NGC 4151 has a mass of about 50 million solar masses. [David W. Hogg, Michael R. Blan- ton, and the Sloan Digital Sky Survey Col- laboration] becoming 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, which the researchers used as the ruler. How- ever, the apparent size of the Eye of Sauron’s ring is so small, the obser- vations were carried out using the Keck Interferometer, which com- bines Keck Observatory’s twin 10- meter telescopes — already the largest telescopes on Earth — to achieve the resolving power of an 85m telescope. To measure the phys- ical size of the dusty ring, the re- searchers 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 a distance to NGC 4151. “One of the key findings is that the distance de- termined in this new fashion is quite precise — with 90 percent ac- curacy,” Hoenig said. “In fact, this method, based on simple geometri- cal 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 AGN. The goal is to establish precise distances to a dozen galaxies using this technique and use them to con- strain cosmological parameters to within few per cent. Combined with other measurements, this will pro- vide a better understanding of the history of expansion of our uni- verse. (The W. M. Keck Observatory operates the most scientifically pro- ductive telescopes on Earth. The two, 10-meter optical/infrared telescopes near the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, inte- gral-field spectrographs and world- leading laser guide star adaptive op- tics systems.) n