An article published in “The Astrophysical Journal” reports a study of over 2,000 active galactic nuclei (AGN) among which some may have two supermassive black holes. A team of researchers led by Pablo Peñil, a Ph.D. student at Universidad Complutense de Madrid in Spain, analyzed the data collected in nine years of observations from NASA’s Fermi space telescope to identify gamma-ray emissions that repeat every two years and could indicate the interaction of two supermassive black holes. 11 of the galaxies examined have nuclei with this type of emissions while 13 others show hints of that type of emissions and require follow-up observations to verify their nature.
Normally, a galaxy has a supermassive black hole at its center, which can have a mass millions or even billions of times the Sun’s. When such an object is surrounded by great amounts of gas and dust, it can heat them to the point that they generate very strong electromagnetic emissions. These are active galactic nuclei, which are the brightest objects in the universe.
During its mission, the Fermi space telescope detected gamma-ray emissions from a large number of active galactic nuclei thanks to its Large Area Telescope (LAT) instrument. These data were analyzed by the team led by Pablo Peñil to find traces of anomalous emissions that could be due to the interaction between two supermassive black holes within a single galaxy. Such a thing can happen when two galaxies merge and their black holes remain separate for many millions of years during which they can interact. The image (ESA) shows an artistic concept of this type of situation.
The researchers found in 11 of the over 2,000 active galactic nuclei observed by Fermi gamma-ray cycles that repeated every two years or so. There may be different explanations for this phenomenon, and among them there’s the presence of a pair of supermassive black holes that orbit each other. Marco Ajello, an astronomer from the College of Science’s department of physics and astronomy at Clemson University, another of the authors of this research, explained that understanding the relationship of these black holes with their environment will be essential to provide a complete picture of galaxy formation.
In the past, various teams of researchers identified galaxies that could have two supermassive black holes, such as OJ 287, which has a blazar-type active galaxy nucleus. Professor Sara Buson of the University of Würzburg, Germany, another of the authors of this research, mentioned PG 1553+113 and PKS 2155-304, two other blazars in which periodic changes in gamma-ray emissions were detected.
The researchers intend to expand the number of galaxies examined to look for more possible supermassive black hole pairs in other data collected by the Fermi space telescope. 9 of the 11 active galactic nuclei spotted in this research were identified for the first time. 13 more of them show emissions that require follow-up studies to understand if they’re periodic. In short, there’s still a lot of work to do to understand which galaxies contain supermassive black hole pairs.