Clearest understanding yet of the life cycle of supermassive black holes

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The doughnut-shaped ring surrounding many supermassive black holes tells researchers how fast the space object feeds and may change how the black hole is seen from Earth. Credit: ESA/NASA, the AVO project and Paolo Padovani

Researchers are using X-ray telescopes and a new data analysis technique to describe space objects.

According to a study by Dartmouth scientists, black holes with varying light signatures that were once thought to be the same objects seen from different angles are actually at different stages of the life cycle.

This new research on black holes known as ‘active galactic nuclei’, or AGNs, indicates that it definitely points to the need to revise the widely used ‘unified AGN model’ that characterizes supermassive black holes as having all the same properties.

The study provides answers to a nagging space mystery and should allow researchers to create more accurate models of the evolution of the universe and how black holes develop. It was published on July 15 in The Astrophysical Journal,

“These objects have mystified researchers for over half a century,” said Tonima Tasnim Ananna, lead author of the paper and postdoctoral research associate at Dartmouth. “Over time, we have made many assumptions about the physics of these objects. We now know that the properties of obscured black holes are significantly different from the properties of AGNs that are not as strongly hidden.

Tonima Tasnim Ananna

Tonima Tasnim Ananna, postdoctoral research associate at Dartmouth College. Credit: Robert Gill/Dartmouth College

Supermassive black holes are thought to reside at the center of almost all major galaxies, including our own, the Milky Way. Gravitationally strong objects devour galactic gas, dust and stars, and they can become heavier than smaller galaxies.

For decades, astronomers have been interested in the light signatures of active galactic nuclei, a type of supermassive black hole which is “accumulating” or which is in a phase of rapid growth.

Beginning in the late 1980s, astrophysicists realized that light signatures from space ranging from radio wavelengths to X-rays could be attributed to AGNs. Objects were believed to typically have a doughnut-shaped ring – or “torus” – of gas and dust around them. The differing brightness and colors associated with objects were thought to be a result of the angle from which they were viewed, which would affect the amount of torus obscuring the view.

From there, the unified theory of AGNs became the dominant view. The theory states that if a black hole is viewed through its torus, it should appear faint. If viewed from below or above the ring, it should appear shiny. According to the current study, however, previous research relied too heavily on data from less obscure objects and biased search results.

The new study focuses on how quickly black holes feed on space matter, or their accretion rates. The research found that the rate of accretion does not depend on the mass of a black hole, it varies greatly depending on its obscuration by the ring of gas and dust.

Tonima Tasnim Ananna and Ryan Hickox

Tonima Tasnim Ananna, postdoctoral research associate at Dartmouth College, and Ryan Hickox, professor of physics and astronomy. Credit: Robert Gill/Dartmouth College

“This supports the idea that the torus structures around black holes are not all the same,” said Ryan Hickox, professor of physics and astronomy and co-author of the study. “There is a relationship between the structure and the way it develops.”

The result shows that the amount of dust and gas surrounding an AGN is directly related to its feeding amount, confirming that there are differences beyond orientation between different AGN populations. When a black hole accumulates at a high rate, the energy blows out dust and gas. As a result, it is more likely to be clear and appear brighter. Conversely, a less active AGN is surrounded by a denser torus and appears weaker.

“In the past, it was unclear exactly how the population of obfuscated AGNs varied from its more easily observable, non-obfuscated counterparts,” Ananna said. “This new research definitely shows a fundamental difference between the two populations that goes beyond the angle of vision.”

The study stems from a decade-long analysis of nearby AGNs detected by Swift-BAT, a high-energy " data-gt-translate-attributes="[{" attribute="">Nasa X-ray telescope. The telescope allows researchers to scan the local universe to detect obfuscated and unobfuscated AGNs.

The research is the result of an international scientific collaboration – the BAT AGN Spectroscopic Survey (BASS) – which has been working for more than a decade to collect and analyze optical/infrared spectroscopy for the AGN observed by Swift BAT.

“We have never had such a large sample of local AGN obscured detected X-rays before,” Ananna said. “This is a big win for high-energy X-ray telescopes.”

The paper builds on previous research by the research team analyzing AGNs. For the study, Ananna developed a computational technique to assess the effect of obscuring matter on the observed properties of black holes, and analyzed data collected by the larger research team using this technique.

According to the article, by knowing the mass of a black hole and how fast it feeds, researchers can determine when most supermassive black holes have undergone most of their growth, providing valuable information about the evolution of black holes and the universe.

“One of the biggest questions in our field is where do supermassive black holes come from,” Hickox said. “This research provides a vital piece that can help us answer this question and I expect it to become a gold standard for this research discipline.”

Future research could include a focus on wavelengths that allow the team to search beyond the local universe. In the shorter term, the team would like to understand what triggers AGNs to go into high accreting mode and how long it takes fast accreting AGNs to go from heavily obscured to unobstructed.

Reference: “BASS. XXX. Distribution functions of Eddington DR2 ratios, black hole masses and X-ray luminosities” by Tonima Tasnim Ananna, Anna K. Weigel, Benny Trakhtenbrot, Michael J. Koss, C. Megan Urry, Claudio Ricci, Ryan C Hickox, Ezequiel Treister, Franz E. Bauer, Yoshihiro Ueda, Richard Mushotzky, Federica Ricci, Kyuseok Oh, Julian E. Mejía-Restrepo, Jakob Den Brok, Daniel Stern, Meredith C. Powell, Turgay Caglar, Kohei Ichikawa, O. Ivy Wong, Fiona A Harrison and Kevin Schawinski, July 15, 2022, The Astrophysical Journal.
DOI: 10.3847/1538-4365/ac5b64

Researchers contributing to the study include Benny Trakhtenbrot, Tel Aviv University; Claudia Megan Urry Yale University; and Mike Koss of Eureka Scientific.


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