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The measurement of cosmological distances: The Eye Of Sauron


One of the most important problems in astronomy is the one concerning the measurement of cosmological distances. Today, however, researchers from the Niels Bohr showed that it is possible to obtain the distance to which is located a galaxy with great precision thanks to a method that is based on the observation of supermassive black holes. The results of this study were published in Nature.NGC_4151_large

Source: NASA

The active galaxy NGC 4151, also known as “The Eye of Sauron” because of its resemblance to the evil eye that appears in the film of the saga “The Lord of the Rings”, is a spiral galaxy rather modest. At its core lies a supermassive black hole still active which increases gas clouds as a result of a process that, according to the authors of a study published in Nature, makes it possible to measure the distance at which the galaxy is located with much greater accuracy to what permits the analysis of the redshift.

“When gas falls toward the black hole, it is heated and emits ultraviolet radiation. This heats a dust ring, which orbits the black hole at great distances, and this heat causes the powder then re-emits infrared radiation,” explains Darach Watson, associate professor of the Dark Cosmology Centre at the Niels Bohr Institute of University of Copenhagen and co-author of the article. “Using ground-based telescopes, we can measure the time delay between the ultraviolet radiation emitted by the black hole and the infrared radiation emitted by the dust cloud later. This delay time is about 30 days and since we know the value of the speed of light, we can calculate the actual physical distance that separates the black hole ring of dust orbiting”.
Using the technique of interferometry, which allows to combine the light of the two telescopes of 10m Keck Observatory in Mauna Kea, Hawaii, the researchers were able to achieve an angular resolution of actual equivalent to that of a single telescope that has a primary mirror diameter of 85m (the distance between the two telescopes). This provides a high power exploratory 100 times better than that achieved by the Hubble Space Telescope, allowing it to measure the angle that the dust disk shape relative to the plane of the sky (about 12 millionths of a degree).
Then combining the angular size of the disk of dust and its actual distance from the black hole, which is 30 light-days, you can calculate the distance at which the galaxy is located applying simple geometric considerations. “We found that the distance is 62 years light. The previous calculations based on redshift fell over a range of values between 13 and 95 light years, going from a huge uncertainty to a very accurate determination. This result is very important in the context of astronomical calculations concerning the cosmic distances, “says Watson.

Radio image - ngc 4151

Source: NASA

Image description: Left: Radio image of neutral hydrogen gas in the spiral Seyfert host galaxy NGC 4151 (Mundell et al. 1999); Right: composite image of the central regions of NGC 4151 showing a 1.4-GHz radio image of the well-collimated plasma jet surrounded by an obscuring torus of molecular hydrogen imaged at 2.2 µm (Fernandez et al. 1999) and an inferred inner ring of neutral hydrogen from absorption measurements (Mundell et al. 2002).

The study was conducted as a result of collaboration between Darach Watson and Sebastian Hönig of the University of Southampton, the lead author. The researchers were stunned by their results. “Our calculations were obtained almost by magic. The most important thing when measuring distances in astronomy must be the high precision, which implies the accuracy of the method. We knew that if the uncertainty was lowered to about 10%, this would have been very significant despite we had no idea how to do it, “says Watson.
“When calculating the mass of the supermassive black hole that resides in the nuclei of active galaxies we must consider two main factors: the speed of rotation and the distance of the stars from the central black hole,” continued Watson. “The speed of rotation can be observed whereas the distance of the stars from the black hole can now be determined more accurately by using this new approach. Our calculations suggest that black supermassive holes weigh 40% more than assumed earlier, a result that fundamentally changes the determination of the masses of black holes. ”
Finally, according to the researchers these more accurate measurements of the distance may be the most thorough method to measure even the Hubble constant, which is the parameter that expresses the rate of expansion and ultimately the age of the Universe.

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