Black holes with different light signatures but thought to be the same object when viewed from different angles are actually in different stages of life cycles, according to a study led by Dartmouth researchers.
The study of black holes, known as “active galactic nuclei,” or AGN, says it conclusively shows the need to revise the widely used “unified AGN model,” which characterizes supermassive black holes as all having the same properties.
The research, published in The Astrophysical Journal, provides answers to a vexing cosmic mystery and should allow researchers to create more precise models of the evolution of the universe and how black holes develop.
“These objects have puzzled researchers for more than half a century,” said Tonima Tasnim Anana, a postdoctoral researcher at Dartmouth and lead author of the report. “Over time, we’ve made a lot of assumptions about the physics of these objects. We now know that the properties of obscured black holes are significantly different from the properties of AGN, which are not so heavily obscured.’
Supermassive black holes are believed to be at the center of almost all large galaxies, including the Milky Way. The objects engulf galactic gas, dust and stars and can become heavier than small galaxies.
For decades, researchers have been interested in the light signatures of active galactic nuclei, a type of supermassive black hole that is “accreting,” or undergoing rapid growth.
In the late 1980s, astronomers realized that light signals coming from space, ranging from radio wavelengths to X-rays, could be attributed to AGN. The objects are thought to typically have a doughnut-shaped ring – or “torus” – of gas and dust around them. The varying brightness and colors associated with the objects are thought to result from the angle from which they were observed and how much of the torus obscures the view.
From this, the unified theory of AGN became the prevailing understanding. The theory is that if a black hole is viewed through its core, it should appear faint. If viewed from below or above the ring, it should appear bright. According to the current study, however, past research has relied too heavily on data from the less obscured objects and skewed research results.
The new research focuses on how quickly black holes feed on cosmic matter, or their accretion rates. The research found that the accretion rate does not depend on the mass of a black hole, it varies greatly depending on how much it is hidden by the ring of gas and dust.
“This supports the idea that the dust structures around black holes are not uniform,” said Ryan Hickox, professor of physics and astronomy and co-author of the study. “There’s a relationship between the structure and the way it grows.”
The result shows that the amount of dust and gas around an AGN is directly related to how much it feeds, confirming that there are differences beyond orientation between different AGN populations. When a black hole accretes at high speed, the energy blows away dust and gas. As a result, it is more likely to be undarkened and appear brighter. Conversely, a less active AGN is surrounded by denser dust and appears fainter.
“In the past, it was not certain how the hidden population of AGN differed from their more easily visible, unobscured counterparts,” Annana said. “This new research definitively shows a fundamental difference between the two populations that goes beyond the visual angle.”
The study stems from a decade-long analysis of nearby AGNs detected by Swift-BAT, NASA’s high-energy X-ray telescope. The telescope allows researchers to scan the local universe to detect obscured and unobscured 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 AGN observed by Swift BAT.
“Never before have we had such a large sample of X-ray detected obscured local AGNs,” Annana said. “This is a big win for high-energy X-ray telescopes.”
The paper builds on previous research by the research team analyzing AGN. For the study, Ananna developed a computational technique to estimate the effect of obscuring matter on the observed properties of black holes and analyzed data collected by a wider research team using this technique.
According to the paper, by knowing the mass of the black hole and how fast it is feeding, researchers can determine when most supermassive black holes have undergone most of their growth, thereby providing valuable information about the evolution of black holes and the universe.
“One of the biggest questions in our field is where supermassive black holes come from,” Hickox said. “This study provides a critical piece that can help us answer this question, and I expect it to become a touchstone for this research discipline.”
Future research could include focusing on wavelengths that allow the team to search beyond the local universe. More closely, the team would like to understand what triggers AGNs to go into a high accretion regime and how long it takes for rapidly accreting AGNs to go from highly obscured to unobscured.
Researchers who contributed to the study include Benny Trachtenbrot, Tel Aviv University; Claudia Megan Erie, Yale University; and Mike Koss of Eureka Scientific.
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