First Mass Estimations Of Faraway Supermassive Black Holes

Artist's impression of a supermassive black hole. ESO, ESA/Hubble, M. Kornmesser

An international group of astronomers has undertaken the ambitious goal of measuring the mass of hundreds of supermassive black holes in the far-off universe. The task is still mostly underway but it has already born some fruits, 44 to be exact.

The team has used the Sloan Digital Sky Survey (SDSS) to observe these 44 supermassive black holes, located between 2 and 8 billion light-years from Earth. The mass of these objects ranges from 5 million times the Sun to a whopping 1.5 billion times. These observations are reported in the Astrophysical Journal.

“This is the first time that we have directly measured masses for so many supermassive black holes so far away,” lead author Dr Catherine Grier, from the Pennsylvania State University, said in a statement. “These new measurements, and future measurements like them, will provide vital information for people studying how galaxies grow and evolve throughout cosmic time.”

Obtaining the mass of these 44 supermassive black holes was possible thanks to a technique called reverberation mapping. As light moves away from a black hole, it reverberates on the galaxy’s gas. By comparing the time delays between gas near the core and gas further out, researchers can understand the dynamic of the system and estimate the black hole mass.

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Artist's impression of the reverberation mapping technique. Nahks Tr’Ehnl (www.nahks.com)/Catherine Grier /SDSS collaboration

Extracting precise estimates from these observations requires a lot of data collected over a number of years. So the team is already monitoring 850 distant quasars. Quasars are far away galaxies with an incredibly active supermassive black hole at their center. These quasar observations need careful calibration as they are very faint. 

“Getting observations of quasars over multiple years is crucial to obtain good measurements,” explained Yue Shen, an assistant professor at the University of Illinois and Principal Investigator of the SDSS Reverberation Mapping 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.”

This research shows the potential for using this technique on a wider scale and the next phase of the SDSS science program will do just that. Beginning in 2020, SDSS will include the Black Hole Mapper project, which will look at 1,000 more quasars.

“The Black Hole Mapper will let us move into the age of supermassive black hole reverberation mapping on a true industrial scale,” added Professor Niel Brandt from the Pennsylvania State University. “We will learn more about these mysterious objects than ever before.”

Supermassive black holes are a key player in the evolution of galaxies and finding out their properties is extremely important.

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