UNIVERSITY PARK, Pa. — A team of astronomers from the Sloan Digital Sky Survey (SDSS), including several Penn State scientists, announced new measurements of the masses of a large sample of supermassive black holes far beyond the local universe.
The results, being presented at the American Astronomical Society (AAS) meeting in National Harbor, Maryland, and published in the Astrophysical Journal, represent a major step forward in our ability to measure supermassive black hole masses in large numbers of distant quasars and galaxies.
"This is the first time that we have directly measured masses for so many supermassive black holes so far away," said Catherine Grier, a postdoctoral fellow at Penn State and the lead author of this work. "These new measurements, and future measurements like them, will provide vital information for people studying how galaxies grow and evolve throughout cosmic time."
Supermassive black holes are found in the centers of nearly every large galaxy, including those in the farthest reaches of the universe. Their gravitational pull is so great that nearby dust and gas is inexorably drawn in. The infalling material heats up to such high temperatures that it glows brightly enough to be seen all the way across the universe, forming bright disks of hot gas known as quasars. By studying quasars, scientists learn not only about supermassive black holes, but also about the distant galaxies that they live in. But to do all of this requires measurements of the properties of the supermassive black holes, most importantly their masses.
“The problem is that measuring the masses of supermassive black holes is a daunting task,” said Grier. “Astronomers measure supermassive black hole masses in nearby galaxies by observing groups of stars and gas near the galaxy center. However, these techniques do not work for more distant galaxies, because they are so far away that telescopes cannot resolve their centers.”
Instead, direct mass measurements of supermassive black holes in galaxies farther away are made using a technique called reverberation mapping. Reverberation mapping works by comparing the brightness of light coming from gas very close in to the black hole to the brightness of light coming from fast-moving gas farther out. Changes occurring in the inner region impact the outer region, but light takes time to travel outward, or "reverberate." This reverberation means that there is a time delay between the variations seen in the two regions. By measuring this time delay, astronomers can determine how far out the gas is from the black hole. Knowing that distance allows them to measure the mass of the supermassive black hole — even though they can't see the details of the black hole itself.
Over the past 20 years, astronomers have used reverberation mapping to laboriously measure the masses of around 60 supermassive black holes in nearby active galaxies. But because each measurement requires months of observation, measurements are typically made for only a handful of active galaxies at a time. Using reverberation mapping on quasars, which are farther away, is even more difficult, requiring years of repeated observations. Because of these observational difficulties, astronomers had only successfully used the technique to measure supermassive black hole masses for a handful of more distant quasars — until now.
In this new work, Grier's team has used an industrial-scale application of reverberation mapping with the goal of measuring black hole masses in tens to hundreds of quasars. The key to the success of the SDSS Reverberation Mapping project lies in the SDSS's ability to study many quasars at once — the program is currently observing about 850 quasars simultaneously. But even with the SDSS's powerful telescope, this is a challenging task because these distant quasars are incredibly faint.
"You have to calibrate these measurements very carefully to make sure you really understand what the quasar system is doing," said Jon Trump, assistant professor at the University of Connecticut — previously a Hubble Postdoctoral Fellow at Penn State — and a member of the research team.
Improvements in the calibrations were obtained by also observing the quasars with the Canada-France-Hawaii-Telescope (CFHT) and the Steward Observatory Bok telescope located at Kitt Peak National Observatory over the same observing season. After all of the observations were compiled and the calibration process was completed, the team found reverberation time delays for 44 quasars. They used these time-delay measurements to calculate black hole masses that range from about 5 million to 1.7 billion times the mass of our sun.
"This is a big step forward for quasar science," said Aaron Barth, a professor of astronomy at the University of California, Irvine, who was not involved in the team’s research. "They have shown for the first time that these difficult measurements can be done in mass-production mode."
These new SDSS measurements increase the total number of active galaxies with supermassive black hole mass measurements by about two-thirds, and push the measurements farther back in time to when the universe was only half of its current age. But the team isn't stopping there — they continue to observe these 850 quasars with SDSS, and the additional years of data will allow them to measure black hole masses in even more distant quasars, which have longer time delays that cannot be measured with a single year of data.
"Getting observations of quasars over multiple years is crucial to obtain good measurements," said Yue Shen, an assistant professor at the University of Illinois and principal investigator for the project. "As we continue our project to monitor more and more quasars for years to come, we will be able to better understand how supermassive black holes grow and evolve."
The future of the SDSS holds many more exciting possibilities for using reverberation mapping to measure masses of supermassive black holes across the universe. After the current fourth phase of the SDSS ends in 2020, the fifth phase of the program, SDSS-V, begins. SDSS-V features a new program called the Black Hole Mapper, which plans to measure supermassive black hole masses in more than 1,000 more quasars, pushing farther out into the universe than any reverberation mapping project ever before.
"The Black Hole Mapper will let us move into the age of supermassive black hole reverberation mapping on a true industrial scale," said Niel Brandt, professor of astronomy and astrophysics at Penn State and a longtime member of the SDSS. "We will learn more about these mysterious objects than ever before."
In addition to Grier and Brandt, the SDSS Reverberation Mapping project team at Penn State includes Distinguished Professor of Astronomy and Astrophysics Donald P. Schneider.
About the research
This research was supported by funding from the U.S. National Science Foundation (NSF) and the Penn State Willaman Endowment. The SDSS-RM team also received support from the Alfred P. Sloan Research Fellowship, the NSF, the U.K. Science and Technology Facilities Council, the National Key R&D Program of China, and the National Science Foundation of China.
This work is also based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France–Hawaii Telescope (CFHT), which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.
About the Sloan Digital Sky Survey
Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the participating institutions. SDSS acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS website is www.sdss.org.
SDSS is managed by the Astrophysical Research Consortium for the participating institutions of the SDSS collaboration, including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, the Chilean Participation Group, the French Participation Group, Harvard-Smithsonian Center for Astrophysics, Instituto de Astrofísica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU) / University of Tokyo, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE), National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatório Nacional / MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University.