Matter swirling around supermassive black holes creates bursts of light that “echo” in nearby dust clouds. These travel signals could serve as a new cosmic yardstick.
When you look at the night sky, how do you know if the patches of light you see are bright and distant, or relatively faint and close? One way to find out is to compare the amount of light actually emitted by the object with its brightness. The difference between its true brightness and its apparent brightness reveals the distance of an object from the observer.
Measuring the brightness of a celestial object is difficult, especially with black holes, which do not emit light. But the supermassive black holes found at the center of most galaxies provide a loophole: they often attract a lot of matter around them, forming hot disks that can radiate brightly. Measuring the brightness of a bright disc would allow astronomers to measure the distance from the black hole and the galaxy in which it lives. Distance measurements not only help scientists create a better three-dimensional map of the universe, but can also provide information on how and when objects formed.
This animation shows the events that serve as the basis of an astrophysics technique called “echo mapping”, also known as reverb mapping. In the center is a supermassive black hole surrounded by a disc of material called an accretion disc. As the disc gets brighter, it sometimes releases even brief flashes of visible light. The blue arrows show the light from this flash moving away from the black hole, both towards an observer on Earth and towards a huge donut-shaped structure (called a torus) made of dust. The light is absorbed, causing the powder to heat up and release infrared light. This clearing of dust is a direct response – or, one might say, an “echo” – of the changes taking place in the disc. The red arrows show that this light is moving away from the galaxy, in the same direction as the initial flash of visible light. Thus an observer would see visible light first and (with the right equipment) infrared light later. Credit: NASA/JPL-Caltech
In a new study, astronomers used a technique some have dubbed “echo mapping” to measure the brightness of black hole disks in more than 500 galaxies. Posted in Astrophysical Journal in September 2020, the study adds support to the idea that this approach could be used to measure the distances between Earth and these distant galaxies.
The echo mapping process, also known as reverb mapping, begins when the record overheats plasma (atoms that have lost their electrons) near the black hole become brighter, sometimes even releasing short flashes of visible light (i.e. wavelengths that can be seen by the human eye). That light moves away from the disk and eventually runs into a common feature of most supermassive black hole systems: a huge donut-shaped cloud of dust (also known as a torus). Together, the disk and the torus form a sort of bull’s-eye, with the accretion disk wrapped tightly around the black hole, followed by consecutive rings of slightly colder plasma and gas, and finally the toroid of dust, which forms the widest and outermost ring in the bullseye. When the flash of light from the accretion disk reaches the inner wall of the dusty toroid, the light is absorbed, causing the powder to heat up and release infrared light. This torus illumination is a direct response or, one might say, an “echo” of the changes taking place in the disk.
The distance from the accretion disk inside the dust torus can be enormous – billions or trillions of miles. Even light, traveling at 186,000 miles (300,000 kilometers) per second, can take months or years to pass through. If astronomers can observe both the initial glow of visible light in the accretion disk and the subsequent infrared illuminance in the torus, they can also measure the time it takes for light to travel between these two structures. Since light travels at a standard speed, this information also gives astronomers the distance between the disk and the torus.
Scientists can then use distance measurement to calculate the disc’s brightness and, in theory, its distance from Earth. Here’s how: The temperature in the part of the disk closest to the black hole can reach tens of thousands of degrees, so high that even atoms are torn apart and dust particles cannot form. The heat from the disc also heats the surrounding area, like a bonfire on a cold night. Moving away from the black hole, the temperature gradually decreases.
Astronomers know that dust forms when the temperature drops to around 2,200 degrees Fahrenheit (1,200 Centigrade); the larger the bonfire (or the more energy the disc radiates), the farther away the dust forms. Then measuring the distance between the accretion disk and the torus reveals the energy output of the disk, which is directly proportional to its brightness.
Since light can take months or years to traverse the space between the disk and the torus, astronomers need data spanning decades. The new study is based on nearly two decades of visible light observations of accretion discs of black holes, captured by several ground-based telescopes. The infrared light emitted by the dust was detected by NASA’s Near Earth Object Wide Field Infrared Survey Explorer (NEOWISE), formerly called WISE. The spacecraft surveys the entire sky about once every six months, providing astronomers with repeated opportunities to observe galaxies and look for signs of those bright “echoes.” The study used 14 WISE / NEOWISE sky surveys, collected between 2010 and 2019. In some galaxies, it took more than 10 years for light to cross the distance between the accretion disk and the dust, making them more echoes. long never measured outside the Milky Way galaxy.
Distant galaxies, far away
The idea of using echo mapping to measure the distance from Earth to distant galaxies is not new, but the study takes substantial steps in demonstrating its feasibility. The largest single survey of its kind, the study confirms that echo mapping takes place equally across all galaxies, regardless of variables such as the size of a black hole, which can vary significantly across the universe. But the technique isn’t ready for prime time.
Due to multiple factors, the authors’ distance measurements lack accuracy. In particular, the authors said, they need to understand more about the structure of the inner regions of the dust donut that surrounds the black hole. That structure could affect things like the specific wavelengths of infrared light that the dust emits when the light first reaches it.
WISE data does not cover the full infrared wavelength range, and a larger data set could improve distance measurements. NASA’s Nancy Grace Roman Space Telescope, slated for launch in the mid-2020s, will provide targeted observations in different infrared wavelength ranges. The agency’s next SPHEREx mission (which stands for Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer) will examine the entire sky in multiple infrared wavelengths and may even help improve the technique.
“The beauty of the echo mapping technique is that these supermassive black holes are not going to disappear anytime soon,” said Qian Yang, a researcher at the University of Illinois at Urbana-Champaign and lead author of the study, referring to the fact that the disks black holes can be actively flared for thousands or even millions of years. “So we can measure the dust echoes over and over for the same system to improve the distance measurement.”
Brightness-based distance measurements can already be performed with objects known as “standard candles”, which have a known brightness. One example is a type of exploding star called a type 1a supernova, which played a pivotal role in the discovery of dark energy (the name given to the mysterious driving force behind the accelerating expansion of the universe). Type 1a supernovae all have about the same brightness, so astronomers only need to measure their apparent brightness to calculate their distance from Earth.
With other standard candles, astronomers can measure a property of the object to deduce its specific brightness. This is the case with echo mapping, where each accretion disk is unique but the technique for measuring brightness is the same. There are benefits for astronomers to being able to use multiple standard candles, such as being able to compare distance measurements to confirm them. precisionand each standard candle has strengths and weaknesses.
“Measuring cosmic distances is a fundamental challenge in astronomy, so the possibility of having an ace up your sleeve is very exciting,” said Yue Shen, also a researcher at the University of Illinois at Urbana-Champaign and co-author of article.
Reference: “Dust Reverberation Mapping in Distant Quasars from Optical and Mid-infrared Imaging Surveys” by Qian Yang, Yue Shen, Xin Liu, Michel Aguena, James Annis, Santiago Avila, Manda Banerji, Emmanuel Bertin, David Brooks, David Burke, Aurelio Carnero Rosell, Matias Carrasco Kind, Luiz da Costa, Juan De Vicente, Shantanu Desai, H. Thomas Diehl, Peter Doel, Brenna Flaugher, Pablo Fosalba, Josh Frieman, Juan Garcia-Bellido, David Gerdes, Daniel Gruen, Robert Gruendl, Julia Gschwend, Gaston Gutierrez, Samuel Hinton, Devon L. Hollowood, Klaus Honscheid, Nikolay Kuropatkin, Marcio Maia, Marisa March, Jennifer Marshall, Paul Martini, Peter Melchior, Felipe Menanteau, Ramon Miquel, Francisco Paz-Chinchon, Andrés Plazas Malagón, Kathy Romer, Eusebio Sanchez, Vic Scarpine, Michael Schubnell, Santiago Serrano, Ignacio Sevilla, Mathew Smith, Eric Suchyta, Gregory Tarle, Tamas Norbert Varga and Reese Wilkinson, 1 September 2020, Astrophysical Journal.
DOI: 10.3847 / 1538-4357 / aba59b
Launched in 2009, the WISE probe was put into hibernation in 2011 after completing its main mission. In September 2013, NASA reactivated the spacecraft with the primary goal of scanning objects near the Earth, or NEO, and the mission and spacecraft were renamed NEOWISE. NASA’s Jet Propulsion Laboratory in Southern California managed and operated WISE for NASA’s Science Mission Directorate. The mission was competitively selected under NASA’s Explorers program run by the agency’s Goddard Space Flight Center in Greenbelt, Maryland. NEOWISE is a project of JPL, a division of Caltech and the University of Arizona, supported by NASA’s Planetary Defense Coordination Office.